This is an interesting Condition. It enables a piece to change its colour and/or its type by moving to a game-array square. [I assume it can change to a pawn as well by moving to the 2nd or 7th ranks]. In addition there is the feature illustrated in this problem of creating a King.

I have a suggestion for the Inventor: to keep the Condition pure and self-contained, dispense with the idea of involving the Rex Multiplex Condition. Instead, limit the number of Kings of each colour to one. The Condition could state: “A move creating more than one King of each colour is illegal”.

Whether the number of Kings can be 0, 1, 2, 3 or more is a relatively minor matter. It is a bit of a sideshow. And the mates as illustrated by this problem could be called quirky if not gimmicky. The foregoing detracts from what should be the main feature of the condition – and could be said to spoil things.

To make my point, I think the Definition for Condition could or should be:

When a piece (other than pawns and Kings) move to a square on the 1st or 8th ranks (other than to the game- array squares of the Kings) it changes to the game-array piece of that square.

This is plain and simple.

This would be a wonderful new Condition offering great possibilities – and would be worth including in Popeye.

[The suggestion in my second comment is questionable. If a bKe8 was to be mated the Kamikaze condition principle would have to apply namely: bKa1,wQb2 is mate].