For Texas to hold onto its pre-statehood territory, while possible, it highly unlikely. The area is simply too large. If somehow a larger Texas is created, or even a one slave & one free "two state solution" whereby East Texas and West Texas are created in 1845, then the Compromise of 1850 may just be buttereflied away, which may in turn bring about an earlier Civil War. Regarding said Civil War, how much of "greater" Texas did/could Texas control in the 1850s and how much of it could Union troops assert Federal control over in a crisis?
Also, one tiny bit of "don't forget that..." nitpicking, the Republic of Texas' border only reached the Arkansas River west of the 100th W Meridian.