Nice to be able to solve a difficult one once in a while.
1. king is stalemated
2. a check that forces the king to move allows a discovered check.
3. Checking on the first move options are limited due to the discovered check possibility..
4. Any first move must maintain the stalemate of the king.
5. The first move cannot allow a spite check response.
6. underpromotion of the c-pawn threatens Nxe7++.
7. evaluate all black responses that can defend Nxe7++.
8. every move that stops Nxe7++ allows a mate in another way:
...Bf8 2. Qg5++
...Nc6 2. Nb6++
...Bxe4 2. Ba2++
...Rxe6 2. Rd4++
...Rxd7 2. Rd4++
...Bg5 2. Qxg5++
In my case, I failed to consider the last 2 defensive tries, but they are similar to other moves that I did consider, luckily. One of the things I need to improve on in my games is to not overlook defensive moves that prove to make my attacks unsound.