Having read the Shattered Frontier history, I'd have to say that it's as a fake textbook it's very well-written. As an AH, I leave it up to review. My rough summary of the situation:
* Pre-POD changes: Aroostook War goes badly for the U.S., leading to the Brits getting more of Maine. This coupled with the Panic of 1837 made Martin Van Buren more unpopular.
* Texan Revolution happens mildly differently but the early history Republic of Texas more or less maps to the real one, with both the presidencies of Houston and Lamar happening on schedule.
* Big POD is the 1844 election of Henry Clay instead of Polk, as seen in the excerpt in the previous post. (John Tyler runs third-party and further messes things up for Van Buren). The anti-annexationists win out and Texas remains independent.
* No Mexican-American War, either. However, California north of SoCal goes independent and is later annexed to the U.S.- which, now that I think about it, is weird because there aren't any post-Clay attempts to get Texas. Deseret is the site of a besieged Mormon community, devastated by a punitive Mexican expedition/crusade. Pius IX gives his blessing to the expedition and Provo is burned. Mormons remain at Ogden and rebuild after the Mexicans leave. See above link excerpt for this in detail.
* Balance of states is pretty messed up with California (and Wisconsin) added but without Texas, so Cuba is purchased from Spain. In exchange for the help in making the deal, the British settle the boundary of British Columbia along the Columbia River based on prior HBC claims.
* 1848: Lewis "Missouri Compromise" Cass becomes president after the Free Republicans (formerly Free Soilers) split the Whig vote from Daniel Webster. He tries to settle the free/slave state situation similar to OTL. Bleeding Kansas ensues. Confederacy ensues.
* Confederate forces trounce Federal troops for the first year. Only northern victory is defeating a southern incursion into the Indian Territory. Natives subsequently declares independence but alliance to the U.S., which is grudgingly accepted.
* Brits maintain neutrality, as do Texians. Confederates find that the cotton demand is declining due to competition with Texas as well as Egypt, India, etc. France does not have active to those sources of cotton, however.
* 1852: Seward of the Free Republicans becomes president with the endorsement of General Winfield Scott. After the past few years of rearming, U.S. starts winning in the West. General Zachary Taylor wins a hard-fought victory at Chancellorsville and halts Federal offensive east of the Appalachians.
* French mail picket ship the Vergt is stopped by a Federal vessel to capture two Confederate ambassadors. Diplomatic incident ensues. Confederate President John Hugh Means asks Napoleon III for alliance in exchange for returning the city of New Orleans and its environs (?!?) to France. Louisianans, pissed off at having NoLa sold out to the French, secede from the Confederacy. Probably the most contrived aspect of this timeline.
* 1855: Zachary wins at Gettysburg against the North, backed by reserves from General Twiggs, and from the French. Meanwhile, the Federals lose at Vicksburg (again foiled by French reinforcements) despite capturing Missouri, Arkansas, and much of Tennessee. Stalemate ensues. Draft riots break out in New York, etc. Cajun War happens briefly as Louisiana secedes again in 1856, attempting to join the Republic of Texas.
* 1857: Douglas of the Democrats signs a peace treaty with the South.
Setting begins with map above.
What do you guys think of the plausibility of all of this? Should definitely check out the Shattered Frontier setting if you find a PDF of Aces and Eights, it's really well-written regardless.