Sorry about the late response hzn5pk -- yours was such a good post, I had to mull it over for a while before I could think of a good response. I am very much obliged, and I thank you for it.

On just a couple of points (for now):
Shawn actually had a thought on Texas earlier:
Texas will probably be a free state, since with that many fewer slaves and slave-owners, there wouldn't be good reason to push Santa Anna on it, and who exactly would be fighting hard to annex Texas to the USA if it was free soil?
As it is, I'm still leaning more toward Texas, if it enters the US at all, entering as a free state.
On Florida, IIANM, there were actually more escaped slaves OTL living there, many of them armed, than ongoing slaves. My sense is, if the US annexes FL, they're not going to want to want to risk all out war to impose slavery there, at least not by 1819.
And then there's the strange figure of William Walker -- when he first went on an expedition to Nicaragua, with an army of former 49ers, they were motivated by general expansionism and had no real thoughts on slavery (may actually have leaned more against than pro, I'm not certain).
Anyway, this was in the early 1850's when the north was weary fighting with their southern neighbors over whether their most recent acquisitions, and so were not keen on new territory. The south, conversely, was, particularly if it were slave territory. Walker made a political calculation to improve relations with Washington by getting southerners excited, and legalized slavery in Nicaragua. This made him very unpopular with the Nicaraguans, and his expedition failed.
All this is to say, ITTL, expansionists wouldn't have to worry about this kind of infighting, and would likely would not impose slave laws on an unwilling conquered population -- meaning, absent Cuba or Puerto Rico, most new territory for America would be admitted as free.
And that gets to your thoughts that this could turn into an Ameriwank -- and where I need to think on this even more.
Like I said -- I am much obliged
