Well, no, the treaty says that “New states, of convenient size, not exceeding four in number, in addition to said state of Texas, and having sufficient population, may hereafter, by the consent of said state, be formed out of the territory thereof, which shall be entitled to admission under the provisions of the federal constitution.” Said states can be created only with consent of Congress when the split occurs. A blanket pre-approval is unconstitutional.
You just quoted Congress's approval!
A blanket pre-approval is unconstitutional.
According to what? You just quoted Congress giving its approval to Texas splitting up into smaller states. What other part of the Constitution is this a violation of? Certainly not Article IV, Section III: "New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress."