This is, of course, still a legal impossibility.
If secession were unconstitutional, neither the militia act nor any other federal act would make it any more unconstitutional.
If secession were not unconstitutional, neither the militia act nor any other federal act could make it unconstitutional.
Nothing in the Militia Act is relevant. The Militia Act of 1792 was not amended. It was repealed as explicitly stated in the legislation of 1795.
Thou shalt not secede. It's in there somewhere. </sarcasm>