You are quite welcome
Why, thank you.
Honestly, I think the best POD for a successful Mexican-American alliance would have been if the United States abolished slavery in the beginning, beginning with the passing of Jefferson's version of the
Land Ordinance of 1784, thus stopping the expansion of slavery into Texas.
You are quite welcome.
One thing to keep in mind about Texas is that east of the Balcon, it is (naturally) pretty well wooded and watered, and looks more like the country east and west of the Mississippi than West Texas ... so even if cotton is not king due to a lack of slave labor, there's still going to be a lot of land-hungry US citizens coming down from Tennessee and across from Georgia, Alabama, and Mississippi into Louisiana and Arkansas after the Louisiana Purchase, and then west into Texas.
I think the single largest issue in terms of whether the US or Mexico controls what became Texas and the Cession territories is (given history as it was to 1824 or so)
if and how an independent Mexico can attract European immigrants, and
if they and "Mexican" nationals from the historical heartland of New Spain/Mexica can be induced to move north, and
if said populations can be induced to remain loyal to Mexico.
Spain, after the initial efforts at settlement in Texas and New Mexico in the 1600s and California in the 1700s, never had luck doing the above, for a variety of reasons; whether Mexico could do much better in the (roughly) decade and a half or two they have of independence before the demographics are irrevocably in the US favor is an open question...
Putting aside any other issues, the US, by gaining independence when it did, had about a four-decade-long lead in terms of national consolidation, creating nationalist institutions and identities, building political and economic stability, and attracting immigrants from Europe, over a Mexico that gains its own independence in the 1820s...that is a difficult lead to overcome, and coupled with the simple reality that the US Atlantic and Gulf coats are closer to Europe (and hence its economic and human resources) than Mexico, and I think it is pretty much impossible for Mexico to overcome...
Note the above opinion is based simply on geography and time, not any sort of black legend vs American exceptionalism type of judgment; there are similar dynamics at work in the post-independence histories of (say) Argentina and Chile, or even Uruguay and Paraguay.
Best,