Under the Texas Annexation agreement was a clause that allowed Texas to form itself into as many as five states if it chose, and if congress agreed. That potentially could have added 8 more members to the Senate. Some have suggested that Texas and the South in general were not too hot on this idea because the demographics showed that any new states carved out would have likely voted themselves 'free states' since slavery was heavly concentrated in the cotton region of the Gulf Coast. North and West Texas have very little slavery and most of the settlers there were free soiler farmers and ranchers.
Correct. Except that Congress agreed, when they admitted Texas. It's a done deal, all Texas has to do is their end now, to do the split. Texas doesn't need any further permissions, as I understand it.
Your reading of Texas demography in 1855 is correct. The slaves were heavily concentrated in the "peach bottoms" of the rivers near the coast. According to a lecture I once heard at a Houston Archeological Society meeting, Brazoria County south of Houston was something like 90% black slaves by the outbreak of the Civil War. The slaveholders were relatively few, tended to be men of English stock (remember that the next time you feel inclined to whip up on the Scots-Irish Southerners) in their 40's who'd immigrated from the Carolinas, Georgia, Alabama, and Mississippi. (Jared Groce, the first man to bring any numbers of slaves to Texas, was from Georgia.)
The inhabitants of more northerly and westerly homesteads tended more to be younger, thirtyish Scots-Irish from Missouri, Tennessee, and Kentucky, only one in five owned a slave, and if he did it was a single slave who lived in a dog-run shack with the rest of the family. Their take on slavery and States' rights was rather different from the planters', who owned the legislature: the Scots-Irish were Jacksonian Democrats, equalitarian and narrow. The planters were more Whiggish and more liberal in their deportment (like Gunnar Myrdal) precisely because social distance made them relatively untouchable. Serene in their social inviolability, they received black men through the front door, as Texas historian T. R. Fehrenbach tells us, whereas the hardscrabbles who were truly threatened by bond labor, labored themselves at maintaining social distinctions that it was a luxury of wealth and position to affect to disdain.