Look, I'll be as polite as I can.
Mexico was in an utterly dreadful spot in the early half of the 19th century. It went through a long and exceptionally destructive war of independence, an empire that lasted barely a year, near perpetual political instability (with only one peaceful transfer of power in almost thirty years) and a war with France. Fix even one of these problems and the others will definitely be lessened (if not straight up butterflied, like the Pastry War could very well be), drastically strengthening Mexico's financial and military position.
So please, please stay on the main topic I raised on the OP. Don't lecture me about the inevitability of Manifest Destiny.
That wasn't my intent at all to "lecture". I'll assume the broad strokes otherwise and non-Mexican affairs for America go as expected, or at least more or less with butterflies here and there.
It's getting a Pacific coast since the Oregon Question was settled peacefully and no Mexican War doesn't really change the 49th was pretty much becoming more and more the accepted boundary line once a settlement was reached. This is a bit more notable for the topic as it sounds, because the majority of settlers pushing into the PNW were northerners be it by ship or via the Oregon Trail. This'll already make the writing on the wall more obvious to the south that the balance of power is swinging to the north, and even if you see perhaps certain proposed or OTL states in the south split off from existing ones/admitted earlier for senatorial balance (your West Virginias or Nickajacks, *Oklahoma opened up earlier) the population advantage is still ultimately for the north.
A subtle point in the Civil War of OTL is that it got accelerated by the Mexican Cession and Oregon Country, but like 95% of the fighting still happened east of the 98th meridian, where the majority of the American population was (and is) still located. A Civil War or some form of armed conflict feels inevitable for a USA that managed to keep a parity between north and south for so long even without a Texas or even more western lands to turn into formal slave states, the south won't stand for becoming the junior section after de-facto dominating the government for decades and it'll feel this crunch even more without Texas and with Oregon. I can see the argument that the lack of Texans hurts recruitment and supplies for any rebelling south, but the lack of a Mexican War to cut a lot of officers' teeth on may show up some even more serious incompetency and bungling generalship in any Civil War till they begin learning how to command properly. Mostly the north'll need someone willing to fight as in OTL, be that Grant or whoever else, since it still holds all the industry and much greater population and can so grind the south down as in OTL.
You could ironically maybe see some Confederates pull the Brazilian Confederado situation into northern Mexico anyway if only because it's closer, but if they were willing to go that far to Brazil I don't know why that cannot happen either.
If you do get a gold rush in California in some form no doubt many Americans will come in - that rush attracted an international audience and I always feel California's gonna be multiethnic no matter if it's independent or controlled by anyone - but many more of them will move back home once the rush is done with since presumably more Mexicans will be rushing up north to a Mexican California. I suppose in the long term also more of what would be Texians and 49ers will instead stay in the Midwest, Upper South, or congregate on the Great Plains more, especially the Oregon Trail route for a continuous band of trade, travel, and settlement across the country. A lot of energy and economics that was put into the west coast over history will instead remain in the old 1819 borders, and the inland/flyover area of the USA'll probably be a good chunk better off with that than OTL's.
