Update: Committing the perfect crime is so much easier when one has the fine lawyers of the World Federation for Chess Composition on one's side:
Regicide score = number of accessories (no witnesses):
17
the unique (!) mate is gxh8Q / gxh8B.
End of update.
Exploiting the fact that kings are exempt from the removal test
we can construct a discovered checkmate
with
21
accessories and
2
witnesses for a regicide score of
15.
unique checkmate is 1. Kd3#
Some more explanation for casual players (pros will want to skip this):
The checkmate is delivered by the bishop in the far corner (h8), let's call him the assassin. It comes as a discovered check, i.e. in the diagram the assassin's path is blocked by the white king on d4 and only once the king has stepped aside to d3 the path to the black king is opened up.
Just as the white king can block the assassin's path, so could most other pieces. So let's skip forward and assume the white king as moved out of the way. The two witnesses, the queens on c8 and h3, could both save their king by moving to block the assassins path on c3, if they weren't themselves blocked by the bishops on c7 and g3. Which makes these two bishops complicit in their sovereign's demise.
What's more, either of those bishops could atone for their treachery by body blocking the assassin on e5 if they weren't blocked by the rooks on d6 and f4. These rooks in turn also could block the diagonal etc. We see that everybody in the north east of the board have blood on their hands except for the two witnesses. The sole reason for all the chaining (and branching) of A is blocked by B is blocked by C etc. is to achieve a high assassin to witness ratio.
The queens on a3 and c1 are similar to the witnesses in that they could block the assassin on c3 if they had clear sight of c3, but they also take away two escape squares for their king, making them accomplices.
