Glad to see all your arguments supporting you position!
I already stated that they did so AFTER they had seceeded - so that in an of itself can not be used as justification. Instead you must argue that the secession itself was invalid.
There seems to be a lot of disagreement over whether states had/have this “right” - repeating your “were, are and shall remain” does not make it more correct. I’ll gladly discuss this with you if your willing to actually bring some arguments to the table ...
That is my argument. That law and legitimacy is ultimately determined by force of arms. The South lost on both counts. They were (and would remain) in violation of the law and the Constitution, and because of their military losses they will remain subject to the Constitution.
A state has no power to declare themselves independent.