Do you have an example of a contemporaneous legislative body articulating a position that secession is contrary to the US Constitution?
Do you have any actual proof at all that secession was a violation of the US Constitution? Anything from anyone in a position of authority?
It seems to me that there were two contemporaneous US presidents who believed that there was no specific enumerated prohibition against secession.
It seems to me that one of those presidents said that he was powerless to stop the various secessions, but he did recognize that it was his duty and that he had specific authorizations and powers to deal with law breaking insurrectionists.
When they committed acts of violence and war against the United States he was honor-bound to respond. It wasn’t so much about what the rebels did as it was how they did it...