From your post --

Lincoln said, Congress could "secure the people a fair chance to form one [a Constitution] for themselves, and then to accept, or reject it, when they ask admission into the Union."

Congress did not dictate to Texas what kind of state constitution to create. However, certain Congressmen worked to reject the admission of Texas into the Union.

That's a fair statement. And the topic is not leaving the Union, but entering it. That's another subject.

Not everything Lincoln said was wrong.