Well now, suppose the USA somehow acquired political control of all of OTL Mexico; I can hardly see how that could happen much earlier than the OTL War With Mexico.
I don't see the cultural assimilation of Spanish-speaking Mexico in total to even the largely Hispanicized "Spanglish" speaking scenario; Spanish-speaking populations would largely hold out longer until countervailing political and social trends give them some leverage to persist and come back.
For instance, slavery was abolished in Mexico by the 1840s; presumably Anglos, whose immigration and hegemony might be largely Southern, would seek to re-impose it, meanwhile taking advantage of established relations of peonage and the like to spread their hegemony over Mexicans. But this would be resented! Say the biting off and chewing of this vast empire delays the Civil War some decades; I still suspect that eventually there will be some new coalition party a la the OTL Republicans who champion abolitionism, and I suspect there is a good chance the coalition winds up capitalizing on Mexican resentment of the greater slavocracy to recruit lots of ethnic Mexicans to the alt-Republican banner; when the nation splits into civil war the secessionists fail to keep control of much of Mexico which sides with the Union analogously to West Virginia--but with a strong Spanish accent! Assuming as I would tend to do that when the dust settles you have a Unionist, nominally abolitionist, victory--certainly if they are going to not only emancipate but enfranchise African-American slaves, they can enfranchise large sweeps of Latino populations while they are at it, as part of consolidating the Reconstructionist Union. After that, while the social and cultural fortunes of Latinos might fluctuate downward again in reactionary eras, I doubt they can sink so low in their demographic strongholds as African-Americans were forced OTL, and they might well indeed serve to check many of the excesses of Jim Crow, certainly on historically Mexican soil. Toleration, even embrace, of Latino culture might become part of the package of typical American progressive, pro-labor politics and Latinos an indispensable part of any progressive coalition. Meanwhile, there are reactionary Latinos a-plenty; the conservative parties would learn to value them too.
I'd expect you'd wind up with a strongly bilingual USA, with bastions of essentially "pure" Spanish-speakers and not much larger, demographically speaking, "pure" English speakers, but with the broad sweep of the population speaking a whole spectrum of "Spanglishes."
Actually there is a wild card here--how thoroughly Hispanicized, in language and cultural transformation, were Mexico's Native "Indians" as of say 1840 OTL? If actually a significant part of the population had largely held out against Spanish language and culture as of that point, possibly ITTL they would never pick up any more of it, as with the Philippines where the limited depth and breadth of Spanish acculturation leave modern Filipinos as much or more influenced by American society as Spanish. I guess that it would be harder for a diverse bunch of Indian languages to be broadly accepted and spread throughout the melange society of the USA and so linguistically that might tip the balance more strongly toward English.
But only if as I suggested above, there are countervailing trends to the expected Anglo bigotry!
If political and social trends of inclusion fail to materialize, either we'd wind up with some dystopic white-supremacist authoritarian empire and the splitting off of large sections from the Mexican "Raj" both north and south, or it would be wracked by separatist insurgencies and an eventual hotbed of radical-leftism that might lead to some kind of federation of North American People's Republics. Or the dystopian fascist empire above, forming at a later date rather than continuous with the original "slavocracy/patron" regime.
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As for the suggestion that the English might have gotten the jump on the Spanish and been the initial European conquerors. well first of all I'm not sure how logistically plausible that might be. Say the English wind up conquering the Aztec/Maya zones but the Spanish manage to get South America.
Even as one of two rival Empires of the Indies, English history is going to be massively butterflied! I'd think that with an early lock on Mexican gold and other profitable tribute goods, the English monarchs would never go Protestant, they'd be so powerful within European councils that they wouldn't want to let go of their influence on the Catholic Church, dynastic schemes to wind up Holy Roman Emperor or controlling the Papacy or so forth. Would that divert them from their OTL path of developing an unrivalled Royal Navy and merchant marine, then being one of the founding nations of modern industrial capitalism?
Meanwhile in America, I'd think that English demographic expansion would be diverted far to the south; you'd wind up with more or less Anglicized Mexico, but someone else would lay claim to the North American Atlantic seaboard while the English were preoccupied to the south. Possibly they'd be able to deny other powers a base there, with an eye to keeping their Gulf Stream eastward trade route back to Blighty open, but probably then neglect or at least delay to settle there.
I'd think there would be a good chance we'd wind up with an England that resembles OTL Spain in historical trajectory and social makeup, and an American Anglosphere that corresponds socially to OTL's Hispanic-dominated but heavily Native-influenced and economically limited Mexico, and either North America would have a status comparable to Africa OTL, or more likely some other European settler colonies (but who?) wind up grabbing that coastline.