Marshall never said that the Constitution was not subject to amendment, any amendment.
That is a new one for me. Have a link?
The principles underlying the Constitution are not law and are frequently ignored. Freedom is abandoned for socialism and it's collection of entitlements, imaginary rights and other nonsense. There is little to nothing in the Constitution protecting the principles underlying the doc. In fact everything you do of consequence today is a friggin' licensed priveledge and not honored as a right. You can't even fix your damn toilet w/o a license, certified and accepted plans, and a friggin' permit. Come election time, you'll have to hire a lawyer and a team of bureaucrats to speak out and be heard, to make sure the jackboots don't end up with justification to toss yer butt in the dungeon.
"Theoretically, the SCOTUS could 'strike down' an Amendment as unconstitutional. -- And that exact point was argued before them in 1919, in a move to nullify the 18th."
There's no limit whatsoever to what mads can be made. Once they're made, they're in the Constitution and supercede whatever was there prior to that. The Amendment overrules, or adds to prior entries. That's why it's called amending.
Wrong. Amendments can change any aspect of the Constitution. There are no limitations on the amendment process.
"Marshall said as much in M v M, -- that any law 'repugnant', - is null & void."
A Constitutional Amendment is not a "law". It is a completely different animal.
"Theoretically, the SCOTUS could 'strike down' an Amendment as unconstitutional. -- And that exact point was argued before them in 1919, in a move to nullify the 18th."
Nope, 'fraid not. Amendments trump the SCOTUS, hands down.