Spain handed over Louisiana to France in large part to curry favor with them. They will have no impetus to sell to the Americans, other than simply to get cold hard cash. Spain's finances were in dire shape, but I still don't think such a sale would happen. Unlike France, Spain has valuable colonies bordering Louisiana territory.
Aye, the buffer zone that Louisiana provides to Spanish Mexico would be no small strategic benefit.
That said, the problem with Louisiana is that it is even larger and less governable than Texas was for Mexico IOTL, and the right of deposit is an issue the United States
will go to war over if push comes to shove. I do agree in that I don't think Spain would sell Louisiana quite as readily as France (which was dying to pawn the place off for money to finance war in Europe), but ultimately the growing shift of demographics in the American favor will likely force Spain to save face by selling the territory rather than risking war. Spain can't really control the vast territory, and America really needs New Orleans, which will make it an issue sooner rather than later. IMHO far more likely to be resolved violently compared to the fairly quiet affair of OTL.
Likely a diplomatic agreement selling the territory in exchange for a bevy of diplomatic promises about settling Mexico that the Americans will agree to but never actually enforce.
OTL, Spain was extremely paranoid about American encroachment onto their New World colonies, even the less valuable ones. They viewed it as Spanish territory and thereby didn't want to give it up - kind of like China and the Tibetan Plateau region today. The Americans were only able to infiltrate the Spanish colonies OTL because Carlos IV sold off Louisiana, Joseph Bonaparte was a bit of a distraction, and Fernando VII was bad all round. Spain only gave up Florida after repeated American filibusters culminating in Jackson's invasion, with Onis beating his head against a wall in Washington - Spain desperately tried to cling to its colonies but couldn't in the end. America will eventually attempt a takeover of Spanish territory which Spain will be unable to resist without British aid - doubtful at best, though it does depend on the circumstances.
A minor nitpick: the Tibetan Plateau actually controls the source of China's water (the Himalayas), in the same way Kashmir does for India and Pakistan, so it's a vital imperative for any Chinese state, communist or otherwise, to control the place.
I guess my big issue is settlers, the American government doesn't really control them (and a confrontation with the American government against American settlers would be an extremely awkward affair, which is likely why nobody would want to risk something like that), and the size of the Louisiana Territory prevents Spain from effectively controlling the influx of settlers. The advent of the steamboat will help farmers and traders along the Mississippi River just as readily as it will help people looking to illegally settle the region. These are probably going to be the hard, rough frontiersman types of OTL who come there and for the first few generations prior to the establishment of major settlements largely live off the land and eat whatever they can shoot.
Armed Americans who are fully capable of disappearing into the untamed wilds settling in a peripheral Spanish territory with only a minimal Spanish presence is unlikely to end well for Spain, to say the least.
Can Britain even really provide significant aid to Spain short of outright intervention? Britain's strategy of containing American expansion by funneling guns to the Iroquois and other British-allied Indian tribes was for all intents and purposes working fairly well in the early days of the American republic. I would think the long-term threat of American expansionism would be a hard thing to sell Parliament on when it came to providing anything in the realm of serious support to Spain.