Is there any way to get player hands working nicely with cards that are mats? I understand that mats make cards non-stacking, except in decks, is that right? And being non-stacking, this prevents the defaults player hand behaviour of cards sent to the same coordinates being displayed in a horizontal, rearrangeable spread, right?
Is the only solution at this time to use a complex system to send each card to a different X coordinate, based on zone occupation or number of cards in hand?
I’m afraid this is the case. As you suggest, Player Hands rely on stacking - each stack of cards displays visually as a row extending rightwards from the stack.
Do you need Mat functionality when the pieces are in Player Hands? If not, perhaps Replace with Other is a solution as pieces move in and out of the Player Hand. However, experience tells me to be wary of this if the module might allow more another player to take an action on the piece simultaneous with it moving in or out of the Player Hand; I suspect that a race condition could result in the piece effectively being cloned.