“secession would be proved.”
Secession isn’t something that needs to be proven. That makes no sense.
Secession is an act of a state. States must allow Congress to prescribe the manner in which a state proves its acts. This wasn't done.
If a state wants to secede they must run it through the Congress to find out how to prove their secession. If a state doesn't want to do this then the state better have the military might to make it stick. The latter was chosen and their military might was insufficient. It was their own fault.
Having a rabble-rousing meeting with a bunch of plantation owners was no way to prove secession. They should have ran it through the Congress to get terms of separation.