Not like they we sexually abusing people or had illegal weapons like grenades

Oh? Is that why so many States were coming and going in the century before the Civil War? Statehood was just a temporary endeavour?

The idea that states didn't leave the union before the civil war cannot be used as evidence for the illegality of secession. You assume that, if secession had been legal, states would frequently come and go. This is probably not true at all, and there is certainly no way to prove such a statement. Not to mention, the secessionist movements that did exist before the civil war seem to suggest that at least some of the population believed secession to be a legitimate option when faced with policies or decisions they disliked.

Is that why no Amendments to the Constitution were made post 1865 that had any effect on the Constitution's supreme arbiters interpretation over the legality of secession?

The Supreme Court declared secession illegal in 1869 (Texas V White).

Was secession illegal? Depends on your interpretation of the constitution. It was probably illegal, and the founding fathers would've certainly intended for it to not occur. But it was not illegal for the reasons you have listed.

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