Again, for about the umpteenth time:
If I move 1. Nc3 and then 2. Nb1. Is this piece move illegal ?
No. But it's not an "un-move". It is a legal move (as described in the rules) that just happens to end up with the piece back where it started.
....and therefore, NOTHING has changed. Which bodes well for the argument, "Did the King ever really move ?"...of which I say yes.
IOW's, one could say one could now 0-0-0 after 0-0 then a X-X (what could be the oppposite of a nomenclature Castling ?) so long as the Q'side Rook hasn't moved.
if you got all your pieces out, could you castle kingside as a fake out, then uncastle, then finally castle queenside with these new rules? Once the king moves you can no longer castle, and castling is a rule, so uncastling would imply that your king has not moved but to uncastle you must first have castled, therefore contradiction.
I think the idea would be fun btw, to take two moves to create an opposite castle attack in a lame middle game.
No. All the castling rules apply EXCEPT you get to hit the rewind button....and only one time. Tho' conceptually I kinda like your idea, but I'm thinking baby steps here.