I started catching up on this with some interest, but I have to say the American war stretches credibility in many many ways.
One point overlooked completely thus far is that OTL, Russia and the USA had very good diplomatic relations. I would think, thinking linearly anyway, that liberalizing Russia in the 1820s would tend only to strengthen that friendship rather than weaken it, it removes a major obstacle.
The true foundation of US/Russian friendship was of course that the two nations had little direct contact, and thus no conflicts of interest (beyond ideology, a point both governments ignored on the diplomatic level of their arms-length relationship). In practice, neither did much to harm or help each other. It was mainly a matter of sentiment, and convenience to the Russians that Britain was being inconvenienced by entanglement in trying to suppress the American Revolution. But US diplomats were grateful and since two of these who went to St. Petersburg were two future US Presidents (John Adams and his son John Quincy Adams--not of course an actual diplomat since he was traveling as the minor son of Adams, but he was old enough to remember personal impressions of Catherine the Great's court) they tended to personally reinforce the friendship.
Now since this was after a rather skimpy relationship, I don't suppose it would form any great obstacle to a sudden reversal, if major Russian and US interests were to conflict. And of course the expansion of the Russian far eastern enterprise that turned into huge claims on North America was, on paper, just such a conflict. But in fact the US presence on the Pacific was a toehold; the Russians were more frustrated by Mexican and British interests there.
I would like the author to trace a more concrete, less nebulous path toward confrontation on the North American Pacific. OTL, the investment of Russia in the American claims was pretty weak. They held Alaska largely by default of rival interests in penetrating there. Vice versa it was not easy to hold there, due to the difficulty of getting food, hence the Fort Ross venture. And it was the threat of the expanding Russian presence that drove the Spanish government of Mexico, known as "New Spain," to start the venture of the Franciscan missions expanding northward to guard the California coast. The missions served mainly as concentrations of Spanish power to repel Russian, or British, adventurers, and in part by denial of Native allies to the Russians, for the Russians like the French of New France worked largely by recruiting Native partners in their trading ventures--with the small numbers of actual Russians who went so very far away from the Russian heartland, they could hardly have operated on an all-Russian basis! Like the French, they worked by tenuous conversion of the Native Americans to Orthodoxy and largely with trade incentives. So, the California missions "dried up the sea" as it were by brutally subjugating the coastal peoples to a highly disruptive authoritarian system that killed them off more than it effectively used their potential labor power. Thus, the Russian venture to seek a site for a food-growing colony was rather weak, resulting in a few dozen people setting up Fort Ross, some hundred miles north of the Bay Area and on a site that was not particularly hospitable. Frankly I think they could have done better with the same people at many sites farther north, including Humboldt Bay.
Meanwhile, of course, Spain, subjugated in the Napoleonic wars (pre-POD here) lost control of Mexico and OTL never got it back. It was Mexican authorities who confronted the Russians at Fort Ross and demanded their withdrawal. (Had the Russians taken Humboldt Bay instead, the Mexicans would have been very ill placed to dislodge them and indeed might have remained unaware of them for a very long time! I have to suppose the Russians were not aware of Humboldt Bay when they picked the Fort Ross site.
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At this point I had recourse to reading more on the history of the Russian-American Company operations and
this historic essay is very enlightening. I learn here that the reason Fort Ross was located so far south was actually a desire to trade with the Spanish holdings rather than to avoid them. And indeed, a diplomatic overture by Baron Wrangel to try to secure and regularize the status of the Russian colony in 1836 came to grief not because of Mexican resistance but because Tsar Nicholas I refused to countenance recognizing the revolutionary republic, which is a point in favor of the overall drift of your timeline, that Russia might indeed make war on Mexico. If they did so though it was not because of a desire to keep Fort Ross; the Russian settlement was not proving to be the producer of agricultural goods that had been hoped a generation before, while the initial attraction of sea otter fur hunting was diminished due to depletion of the stocks, and meanwhile the influx of both Mexican and Anglo settlers from the USA was undercutting the value of Russian trade goods once profitably sold to the Spanish and later Mexican locals while the price of agricultural goods to be obtained from California settlers was rising. Fort Ross was failing, OTL anyway, on economic grounds. In these perspectives a second venture at a place like Humboldt Bay would have been unwise, since the whole point of FR was that its development would be supported by Spanish settlers nearby to trade with, and no such help would be forthcoming so far north, whereas the experience of FR demonstrated that expectations of regional self-development were exaggerated.
Note that the story of the rise and fall of the Russian settlement in Central California overlaps your POD, being started before it and therefore presumably going on, in perhaps modified form, during Tsar Alexanders ATL reforms.
Now, given that Russia and the USA had good relations (and I don't know how OTL Nicholas I squared the self-identity of the USA as a revolutionary, anti-monarchial, democratic republic with his hostility toward these as shown by refusal to recognize Mexico) I'd have thought the Tsar would give some consideration to negotiating his interests in the New World with the USA.
Having read a little more though, it does seem that the apparent good relations between the powers were indeed a tenuous thing, and that the Monroe Doctrine (which it should be noted, was at least as much a British idea as American) was indeed adopted in part aimed against Holy Alliance schemes to restore Spanish power in the New World, and that Nicholas did try to use US good offices to get a green light for the project. I can see his being personally rebuffed in favor of British interests (so recently after the War of 1812 too!) might set him on a less Yankee-friendly course.
The most significant development you've mentioned so far to make Russian actions risking and in the end causing war between the USA and Russia is this matter of General Pestel's ultra-nationalism. The general himself you tell us was a republican and thus mortal enemy of the Romanov dynasty, but in the context of lasting and growing liberal reforms, opinion in Russia is clearly no longer a matter of the Tsar's will alone, not even officially. Clearly a middle ground between Pestelism and OTL conservative Tsarism might exist, in which constitutional loyalists to the dynasty adopt some of his chauvinist notions.
Ironically liberalism, with its nationalist baggage, may do more to poison US/Russian relations than any amount of ultra reactionary Tsarism did OTL!
Granting then the premise that the Russians do scheme, in defiance of the after all weak, unOrthodox, ultra-liberal US Republic's interests, to restore monarchial Spanish Empire to the Americas (presumably, unless I overlooked some details, the main thing Russia gets in the Americas is a strategic and trading partner in Spanish California, and the benefits of a global alliance with a strengthened Spain) and granting that a project that is foolish and ill-run is entirely within the run of normal historic behavior (especially when there is a Romanov or Hapsburg monarch involved, and here we have both!) then I suppose in broad strokes the US-British alliance against it falls into place, and these two Anglo powers, in cooperation, can be expected to win.
That said, I think the outcome of the USA absorbing all of Mexico is profoundly unreasonable. It isn't in the interest of the USA (if we assume that full Constitutional rights are immediately extended to the Mexican people top to bottom, and their states have the full status of US states--and to assume otherwise, that some racist hierarchy whereby only Anglo overlords and hand-picked Castilian-Mexican aristocrats are given full rights and control of their states with the majority being second-class, disfranchised peons is to sow dragon's teeth in the Republic big time) to attempt to swallow all of Mexico, nor will Mexicans, even if instantly and universally accorded full US rights, be very keen on it. And let's not lose sight of the kingmaker position Britain is in. To attempt to deny the republican ambitions of either American nation would be folly, but why should Britain favor the instant doubling of US population, area and power and raise up a bilingual, bi-cultural Federal Union? The alternative is obvious and palatable to all victorious parties--support an independent Mexico comprised of all parts of the old Mexican/New Spanish claims that are actually populated by significant loyal Mexican citizens. In other words, the USA might well lay claim to and be granted the vacant (of European-affiliated people) Indian territories of the interior between California and New Mexico, but these two regions, being settled by Mexicans, should remain with Mexico. I can see a case being made for Mexico to agree (grudgingly) to surrender California, so recently settled and with so many non-Mexican settlers, but not I think New Mexico, which has been part of the New Spanish and Mexican system for centuries.
Wherever the border is drawn, probably close to OTL (though perhaps never exactly where the OTL border is) Mexico should still still exist as a sovereign republic, because the Mexican participants in the anti-Spanish/Russian war can play off the ambitions and interests of both the USA and British Empire against each other, and the USA does not have decisive say because Britain is quite powerful.
I would suggest that the USA gets to annex California down to the north shore of San Francisco Bay and the Sacramento river system, following some branch into the Sierras--this gives it Marin, Sonoma, and the north central valley and all the land north to Oregon, in return for respecting Mexican sovereignty eastward. And that Mexico also loses the Great Basin considerably southward of the Bay's latitude, the river border going to the Sierra Nevada watershed then southward to leave Mexico the watershed of the Colorado River and all mission territory in Arizona and on east to New Mexico, where the border runs south along the Rio Grande to merge with the OTL Texan border. Thus the USA has Colorado and everything north and west, Utah and OTL state of Nevada, but not Arizona or New Mexico, and the Mexicans retain the southern California Central valley and south bay including San Francisco, and thus of course the Los Angeles/San Diego hinterland. US and Mexican citizens have reciprocal rights to trade in the other's territory and even to settle, subject to laws that treat them either as friendly aliens or naturalized citizens; each is responsible for keeping Indians in their territory from bothering anyone on the other side of the border, and of course respecting each other's mineral and other claims. The USA has no more than half claim on the Colorado River waters and on those feeding San Francisco Bay.
From the 1838 point of view, these are still tremendous and valuable gains for the USA; shared access to San Francisco Bay effectively neutralizes the zone as a desirable naval base for either side, unless both are very friendly and trusting of one another. But the US can develop Portland Oregon very aggressively; it is far from both Mexican and British claims. I note and approve that the USA has waived the claim on Puget Sound entirely to Britain, but the border follows the watershed and not the Columbia river shore, so the USA controls the latter river fully and thus can protect the Columbia mouth as a major Pacific port. Southern Oregon now enjoys a tremendous buffer of mountainous (not very desirable) coast down to the Golden Gate, and can put naval bases to safeguard its interests in that port at Mendocino, Bodega Bay and Drake's Bay. Inland, the northern Central Valley is excellent farmland, as are the valleys of Sonoma County and Napa. Gold, when it is discovered, is IIRC mostly north of the proposed border and the mineral wealth of Nevada is all US. So is the settlement-friendly land in the region of the Great Salt Lake. The routes to Oregon are secure, as is a set of routes to northern California.
Mexico on the other hand does not have to attempt to govern, defend or control much territory where Mexicans do not already live. She gets, relative to OTL, half of OTL US California, good control of the Colorado River (enough to veto excessive diversions into US territory and reserve at least half for herself, which enhances the value of northeast Baja California and northwest Sonora) and relative to OTL avoids a great humiliation. She now enjoys British favor, and if that becomes burdensome can court US favor instead, attempting to play both dominant Anglo powers off against each other.
Britain gains full control of the entire Puget sound region and I don't know how much of Russian Alaska, and has the potential to set Mexico against the USA (at the price of offering Mexicans the better deal of course).
Why then should Britons and Mexicans agree to let the USA absorb Mexico completely? If indeed common Mexicans think that the Anglo-Americans are going to let them enjoy full political and human rights alongside white Protestant English speakers, that might induce them to join the Federal Union, although it certainly will be inconvenient for them to be governed from Washington DC--perhaps Mexican influence will get the capital moved far west and south, to New Orleans or a Texan city or perhaps even to the Rio Grande. But I while I don't think the opposition to Papist brown Spanish speakers of mixed Indian/Spanish/African ancestry having equal rights and power will be as universal and monolithic as some assume, I fear it would be strong enough to poison the deal. Look what we did with the Philippines when we conquered them after all! This is many generations earlier, but that just means that if some forms of US bigotry have not had as long to evolve, others (anti-Catholicism for instance) have had less time to decay. If Anglo-Americans proclaim they will welcome the Mexicans in as equals, many will do so with their fingers crossed behind their backs. And I don't think the Mexicans will welcome Union with unmixed joy either.
For instance one issue Texas broke with Santa Anna's Mexican government over was that of slavery. Slavery was illegal in Mexico, full stop, and the Anglo-Texan settlers had to agree to free their slaves (along with adopting Roman Catholicism). Union means suddenly that all the new Mexican states are free to adopt slavery if they wish to. One hopes that the good element of Mexican culture abhorring it prevails, but US Anglo influence from south of the Mason-Dixon line--the part of the USA closest to Mexico after all--will twist arms to legalize slavery. Adoption of all those Mexican states--I count at least 19 on the map, and maybe more--will tip the Senate and Congress powerfully against the slavery interest unless at least half of them adopt slavery. So, either the US southern states will oppose entry of Mexican states and people on equal terms, or will only be mollified 10 or more of these states and half or more of Mexico's population are subject to slavery. Whereupon the northern, abolitionist old Anglo states will be up in arms lest some clever thimblerigging leaves non-slave states in the distinct and permanent minority.
All this is a mess that the USA can avoid simply by being restrained in annexations from the republic they were supposed to be in a war to protect and liberate. It also sidesteps pressure to move the capital, to change US linguistic rules and habits, issues of religious jealousies.
Another stumbling block to the war you've portrayed is Andrew Jackson's role in it. Certainly Jackson can be a charismatic war leader, but he's already earned those laurels. You can explain how Jackson gets a 4th and even 5th term due to being seen as the great hero of this glorious war, but I am scratching my head as to how he got the
third term in which the war is prosecuted. OTL he was persuaded to step aside in favor of Van Buren, and I don't see why he wouldn't do the same here, before the war erupts. Matty Van can after all appoint Jackson supreme general in the war. With the victory under their belts, even on the more reasonable terms I suggest with Mexico still extant and even holding half of California and New Mexico, both have ample glory; Van Buren can reasonably be re-elected, Jackson retires to his final years with a very glorious name indeed. Conceivably after MvB's second term, Jackson runs for and wins a third term, only to die early in office.
The thing is, if you want an Anglo-American alliance, you'd do well to leave Jackson in some other role than President, for the man was a virulent Anglophobe. Martin van Buren can probably finesse that more effectively. You might say, well Jackson's mania is why the USA winds up so very aggrandized, but then we have to ask why does Britain come in and pull American chestnuts out of the fire, only to get nothing from the deal beyond some minor concessions of territory in the northwest and northeast (I do note how truncated Maine winds up being).
What is needed is a more reasonable Presidential succession, and this may open the way for the Anglo-American alliance you want, which ought to point to an outcome where Mexico is no worse off than OTL. As an American idealist, I'd love the idea of Mexican and US citizenship merging in the 1830s, but I don't think it has any realism to speak of unfortunately.
Neither does Jackson being elected to five terms.