1. actually that move is mate, it’s a double-check so you can’t interpose, Kxd8 is illegal since it moves the king into check, you can’t move the king to the 6th rank because of the queen on b6, you can’t move the king to any other square on the 5th rank because of the rook on a7, and you can’t move your king to the 8th rank because of your new Queen on d8.

        1. The problem with d8=Q# is that you can’t move the pawn in the first place. It is pinned by the bishop on c8; the white king is on the other side on h3.