An independent Texas would likely rival Brazil for being the last to abolish slavery, but I think it would be very hard for it to last into the 20th century.
I disagree. The Republic of Texas in 1845 looked very little like the ante-bellum slaveholding State of Texas in 1860. Although slavery grew as an institution during the 10 years that the ROT was in existence, only about 20% of the population of Texas were slaves in 1845. Only after statehood did slavery explode where the number of slaves grew to be over 30% of the population. During the ROT, most of the country was too remote and undeveloped for slaveholding plantations to be workable, and as such, most of the settlers of these regions were non-slaveholding small farmers, craftmen, and tradesmen - and many emigrated from non-slaveholding areas of the U.S. and Europe. While the slaveholding class did hold much of the power in the ROT government, their overarching goal was to be annexed by the U.S. If for some reason, the U.S. declines to annex Texas, and it remains a Republic, the slaveholding class will eventually forfeit their power to either Mexico, A European sponsor state like Britain, or a more frontier/pioneer class of Texan, none of which has as its priority the expansion of slavery. So I see a ROT that somehow, against long odds, endures, it will be a place where slavery does not expand, but likely declines and eventually is killed off.
I think an independent Texas could easily mean an earlier U.S. Civil War, as the Southern planters see the writing on the wall sooner.
Well certainly, the South will attempt to kill off the Missouri Compromise probably as early as 1846, so that they can attempt to create slave states in places like Kansas and Nebraska, where we would probably see much more conflict between slaveholders and free soilers than we did in OTL, which could lead to outright civil war, with or without the formal secession. But any war would be so diffeent than the one in OTL.
West Texas is bad for slavery, but everyone lived in the East, Central, or Valley at that time anyways. In 1847, Texas already had 38,000 slaves for 102,000 whites (counting those of Mexican dissent). It was a major institution.
German immigrants were already, and would continue to be, swamped by pro-slavery settlers from the U.S. South.
Texas doesn't need foreign help to defend itself against Mexico, it only needs a decade or two more of Mexican incompetence, something which is very likely.
Texas wasn't swamped with pro-slavery settlers from the U.S. South until after statehood. During the ROT there were many settlers from the Ohio Valley and the Upper South, as well as thousands of Europeans - Germans, Swiss, Czechs, French, Alcacians, Irish, etc.
Plus - even though I am a diehard Texan, and it may be heresy to say - without annexation, I don't think ROT survives. Regardless of Mexico's incompetence, the ROT had no money, no credit, had huge debts, few friends, little recognition, thousands and thousands of miles of land with little or no ability to protect against invasion or even Indian raids, little infrastructure, and way too much political infighting to survive.