Congress could not have fully consented to the division of the state because it wouldn't have fulfilled its obligation to ensure a republican (little 'r') form of government ("admission under the provisions of the Federal Constitution"). So in order to prevent Texas from dividing into additional states, all Congress has to do is withhold approval of the proposed constitutions of the new states.
Nope, sorry. The first section of the Resolution specifically required the adoption of “a republican form of government.”
Again, your interpretation does not hold up, because it renders the entire section superfluous and utterly meaningless.