Read the decision.
The Constitution contains nothing prohibiting secession; your insistence on differentiating between unilateral v. bilateral is your opinion, which you're welcome to, but can't be defended or advanced beyond the realm of opinion.
And what are you going on except your opinion, and your opinion alone? You cannot point to a section of the Constitution which specifically allows secession any more than I can point to a section which specifically prohibits it. So we both rely on interpretations of various sections to support our position. The difference is that the Supreme Court held the same position that I did, and it ruled that the position you hold is incorrect.
No. The Founders specifically included powers for Congress to oversee the admittance of new states, but not prohibiting secession (unilateral or otherwise). The Tenth Amendment reserves all powers not granted to the federal government to the states. That's not my opinion, its fact, which is more than the Supreme Court displayed in their opinion.