A lot of this depends on the relations between Texas and the European governments. I've read that both Britain and France backed away from a more formal recognition and trade relations because of the US's constant will we/won't we Annexation talk. I think it likely that if the US was cool to cold on Annexation that the British or French would step up their overtures to Texas.
Mexico had a hard time moving 5-6 thousand troops to the Texas border in 1836. Even in 1846 when the Mexican army invaded the disputed territory near Brownsville, they managed to move less than 4000 north of the Rio Grande. I'm not saying that Mexico couldn't move more troops north, but the logistics become harder and harder the larger the force is.
The longer that Texas exists as a Republic, the larger their population could become. Even so, I don't think that Mexico would easily give up its claims on Norte Mexico. But being able to enforce their claims, that's another matter. Read about General Adrian Woll's invasion of Texas in 1842.