Mate in weird number of moves

Alright great job so far, quite impressive!

The "trickier" solution for #9 is that, white was already half way through 0-0-0, he only moved his king, but not his rook. So white mates by -0-0 or Ke1# (yes I know this is a tricky one )

And the other solution for 12# is - black uncastles long (yes, you can uncastle through check in these type of problems), then white retracts fxe6 en passant, checkmate!

Ahhh ok.. both to do with castling ok. cool solutions

It's good to know that I didn't screw up in my logic though :) I just didn't think outside the box enough for 12

This is my attempt at one of the puzzles: Mate in -1.5 moves (undo blacks move first then whites then half a move for black)

Hmm that looks like a cool position.

I'm thinking, black retracts Ra1xe1R, white retracts Nd1, then black retracts half castling by Ke8#, where he is checkmated.

Is this the solution?

yes :) good job. I wanted to do a -1.5 move checkmate and have the position starting from opposite side being checkmated.  You could also do Q instead of R being the peice that was taken and knight could also go to b1.