Or.....First great update and very nice touch of writing about the first salvos of the American civil war and then going away. Awesome planning. Leave everyone in anticipation. Lol
The way things are playing out the biggest threat to Mexico will actually be possible English speaking settlers being discontent with rule from Mexico City. We know that in reality not very king or prime minister will be great snd some just mediocre (hopefully none awful). The huge expanses to reach the northern territories make it hard and expensive to not only settle but to patrol. With part of the territories full of natives who will attack Mexicans or Americans equally. The easiest way to get to Texas and California is by ship. So their future (especially northern territories) not guarantee. You gave also given Mexico a lot of southern territory too. Will we see problems in Yucatan or other iotl Central American countries.
Mexico if not careful could be ripped apart from competing interests.
I can also see that British investors approach Mexico about creating a Canal. The obvious location be the aborted Nicaragua site.
The english speakers could be folks looking to escape the lunacy of the disUnited States and finding a nice home/future in a stable/sane Mexico. The roles are being reversed here: USA is not stable, while Mexico is. The USA experiment is not going so well, so far. NE, almost magically, has bounced back way too quickly from the drubbing they took, but the rump USA has been battered and torn, and now is in the middle of a civil war. No matter how the civil war ends up, we should not expect the country (or countries) to be anything resembling a robust nation worthy of admiration/imitation. So, the setup so far should see the Mexico Northern newcomers content enough to remain with Mexico. If they can remain in this state for a generation or two, a national sense of Mexican identity should emerge, which will survive a less than stellar King. Plus, a generation or two of stability and growth will see the remote expanses shrink, just like it did for the US.
In this TL, look for a lot of immigration (that OTL went to USA) going to the opportunities that await them in a stable Mexico. This will dilute the US migrants.
an OTL Texas scenario is not necessarily a good, or likely, option.
Or, the northerners might ignore the shit show going on beyond their northern borders, and say we can do that better, and secede.