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Indeed, the digestion is most conveniently indicated through changing the piece type to a visibly different one. It is no crime in chess variants to have invisible game state (e.g. castling rights in orthodox Chess), but I think this would just invite errors in the Leaf Pile case.

It is true that 'collapsing' game-theoretically identical piece types into one would affect the repetition rule. But this seems an advantage rather than a disadvantage. What would be the use of prolonging play by allowing another set of repetitions of game states that are considered artificially different (e.g. because two Mummies got swapped)? In the end repeating the sequence of moves will swap the pieces back, with the same result. In my experience pieces almost never get swapped in games without drops. (Jocly considers pieces of the same type distinguishable when testing for repetitions, in deviation of FIDE rules, and it only started to cause problems when I implemented Shogi variants.)

BTW, I am pondering about how to make a more compact description of the rules of this excellent game. A formulation that seems to go a long way would be:

Pieces can change location either by 'Moving', or by being pushed (by a Go Away). In general, Moving is only allowed to empty, unpolluted squares. But there is no restriction on squares pieces can be pushed to; the pushed pieces will coexist there with any previous occupants. Exceptions are the Zombie (which can Move to any square, and then destroys all pieces that were there before), and the Leaf Pile (which can Move to unpolluted squares containing only mobile pieces). Zombies cannot coexist with Ichor, and the two agents destroy each other. Leaf Piles digest all pieces that try to coexist with them; when two Leaf Piles meet the stationary one is digested. Removal of pieces due to failure to coexist happens automatically at the end of any turn.

About the UI issues with Go-Away pushing order: wouldn't it be a natural interface to highlight all adjacent pieces after ordering a push by clicking the Go Away twice (as already has to be done now), and then allow the user to 'click them away' one by one?

And some thoughts about compulsion:

Replacing a Ghast compulsion by a lesser one (at greater distance) was explicity declared to be legal. The other two types of compulsion do not exist in grades. (Although they could, depending on the age of the Ichor or the number of pieces in a crowded square.) It was not specified whether it was legal to replace one type of compulsion by another. E.g. when a piece is pushed from an ichorous square onto a Ghast square, is the compulsion addressed? And when the reverse happens? What if a piece gets pushed to an ichorous Ghast square? Can you resolve that by pushing it to an empty square closer to the Ghast? Or would you have to address all pre-existing compulsions on the same piece simultaneously?

I would be inclined to require that, with the exception of getting a lesser Ghast compulsion, the piece should be free of all compulsions after the move in order to count that move as legal.