This is a follow-up to (Almost) all hands on check by @loopy walt.
The task (mate in one with as many essential pieces as possible) stays the same, but you are to go about it more cunningly.
Recap of the original rules:
Construct a legal position where white to move mates in one, subject to:
The mate is unique as defined in the Handbook of Chess Composition, article 13(2) with footnote 16 (thanks @trolley813) where it says: "[...] except that in the final move a promotion into different pieces having partially the same power (for example queen/rook or queen/bishop) may be tolerated.
Removing any piece except a king results in a legal position (white to move, i.e. white king may be in check, even mate, black king mustn't).
An "accessory to regicide" is any piece which when removed leaves a position where white can no longer mate in one.
A "witness" is a non-king piece that is not an accessory.
Let us define the regicide score as number of accessories - 3 x number of witnesses.
Task: Maximise the regicide score.
Our team of overpaid legal mercenaries have identified a legal loophole which you are to exploit to the best of your ablity:
Handbook of Chess Composition, article 16(2): [...] An en-passant capture on the first move is permitted only if it can be proved that the last move was the double step of the pawn which is to be captured.
Accordingly, you are to construct a position subject to
white to move provably has an e.p. capture that also checkmates the black king.
no other white move is a checkmate.
removal of any single non-king piece yields a legal (white to move) position.
In as many of these positions as possible
it cannot be proved that e.p. is available.
no mate in one is available.
Note: I've tried this strategy a bit and it wipes the floor with what was tried so far. A regicide score in the high 20s should not be a problem. If you feel ambitious aim for 30.
In this example we can prove that b7-b5 is the only last move possible: The king couldn't have moved because all the squares he could have come from are threatened doubly. The knights couldn't have moved because the squares they could have come from would have checked the white king. And the other pawns couldnt have moved because the squares they could have come from are occupied. Note how the pawns on e2 and g2 are crucial accomplices because they prove that the only missing white piece, the light squared bishop died peacefully in his home on f1 and not on the battlefield, meaning that none of the black pawns could have arrived at their position via capture.
What makes this a non-example is that the checkmate is not unique.