Yes/Yes
To those voting no on the first, is that just with regards to unilateral secession?
1. No. I thought this was settled by Texas v. White, 1869. Discussions The Union between Texas and the nation "was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration or revocation, except through revolution or through consent of the States."
I would agree that unilateral secession is in violation of the Constitution. However, could it not be reasoned that the people voting for an independence referendum are in fact voting for revolution? Otherwise, I would just say that it would require an Act of Congress to approve secession.