[4CJ]

The first one is easy. Article I, Section 10, Clause 3: "No State shall, without the Consent of Congress,...enter into any Agreement or Compact with another State, or with a foreign Power..."

A citizen of the state could sell land to a citizen of a foreign state of nation. The state could pass legislation removing the area from it's jurisdiction. No agreement of compact with another state necessary. Borders reduced or enlaged, Congress out of the loop.

While there is nothing in the Constitution that prohibits secession, unilateral actions like leaving the Union would be prohibited since Congressional approval is necessary for changing the status of a state.

Congressional approval is necessary for adding NEW states, forming a NEW state 'within the Jurisdiction of any other State', or forming a NEW state 'by the Junction of two or more States' [a merger], or forming a NEW state from 'parts of States.' There is nothing, like you stated, in the Constitution that prohibits secession.

They are, but more specifically their legislatures are. And legislation passed by Texas in support of secession, and in support of the rebellion, was unconstitutional.

No. The people of each state are not bound by the supremacy clause. They are not ALL 'Members of the several State Legislatures', nor all 'executive and judicial Officers' of the state. The state of Texas did NOT pass legislation seceding from the union, the people of the state of Texas in convention seceded from the union.

[Non-Sequitur]

The state could pass legislation removing the area from it's jurisdiction. No agreement of compact with another state necessary. Borders reduced or enlaged, Congress out of the loop.

Not legally, no. Doing what you suggest would violate the Constitution.

Congressional approval is necessary for adding NEW states, forming a NEW state 'within the Jurisdiction of any other State', or forming a NEW state 'by the Junction of two or more States' [a merger], or forming a NEW state from 'parts of States.' There is nothing, like you stated, in the Constitution that prohibits secession.

Congressional approval is required for any change in status. That would include secession.

No. The people of each state are not bound by the supremacy clause. They are not ALL 'Members of the several State Legislatures', nor all 'executive and judicial Officers' of the state. The state of Texas did NOT pass legislation seceding from the union, the people of the state of Texas in convention seceded from the union.

Yes. The Constitution is the supreme law of the land. Any actions taken by the Texas legislature supporting illegal secession, and an legislation passed supporting the rebellion, were illegal.