"Texas is a free and independent State, subject only to the Constitution of the United States, and the maintenance of our free institutions and the perpetuity of the Union depend upon the preservation of the right of local self-government, unimpaired to all the States."
"Subject only to the Constitution of the United States..." If the Constitution does not allow Texas to withdraw unilaterally then legally she cannot do so, even if the Texas Constitution specifically said she could. Which, I will point out, it does not.