Soverihn & Faeelin-
Let's codify this Spain and New Spain.
My thought in the OP was a New Spain held because Iturbide switches sides and Mexican conservatives never end up favoring independence. So Spain is still a weakened victim of the Napoleonic Wars. The success of Mexican independence depended not only on the weakening of Spain during its occupation, but also Spain's postwar jerking back and forth between liberalism and conservatism which by turns irritated liberals and conservatives in the colonies.
So, in the ATL, it is not the whole Spanish America, spared from the peninsular war, rather this Spain has beaten the rebellions in New Spain in the 1810s and 1820s, but has lost South America, were the rebellions in La Plata and Gran Colombia were more successful earlier in OTL. The success of those rebellions makes more loyalist Peru and upper Peru unviable, but does not make the more accessible New Spain unviable, at least not yet.
This Spain still finds itself having to yield Florida and make a Transcontinental Treaty by 1819.
With all these prerequisites established, will Spain make Empresario grants in 1820s Texas? If it does, will the Texans rebel in the 1830s. And if they do, would the US come to their rescue?
Look, I know the demographics and growing interest are going to make America desperately hungry for California and Texas no later than 1855-1860. And, America would doubtless have the potential to develop a respectable navy to beat the Spanish by then, and indeed up to decade earlier.
The question is if the US could reach the political consensus, despite growing intersectional concern over slavery, to build up the navy to win at Veracruz and Cuba along with the Army to take mainland territories from Spain.
Faeelin, if this goes as you suggest, Spain's victory over the rebels in New Spain bothers America a great deal and she builds a navy and allows territorial disputes over the southwest to come to war by the 1830s, how big a bite out of Spain will this USA take?
Just Texas and the historic Mexican cession?
Texas and the historic Mexican cession and Cuba and Puerto Rico?
Texas and the historic Mexican cession and Cuba and Puerto Rico and the Philippines and Guam?
All that and even more of the north Mexican desert?