I understand that in multiplayer games, if a human slot isn't occupied by a player, then none of its units are spawned on the map, so the slot is essentially wasted. Even if a slot is empty (in this case, let's say Player 2 leaves before the game launches), can units still be spawned in for Player 2 and AI scripts run, essentially turning the empty player slot into a Computer player?

I'd like to make a diplomacy map where empty slots could be taken over by a crude scripted AI so that games could remain interesting even with variable player counts, so that a full house isn't always required.

If the answer is no, I think the EUD emulator still supports writing to player information in memory (IIRC I overwrote the blank Player 9-12 player information with faction names and race flags, although this had no practical impact on the game since these players are totally disabled). I wonder if, even if a player isn't present, if EUDs could be used to just overwrite the slot with Computer flags to maybe permit spawning/trigger execution?

A few quick pointers:

To avoid critical units never appearing, don't preplace units, create them for player x. Trigger owned by a required or cpu player. Alternative have a required or cpu player slot that initially owns all units then give them to the players.

You can detect if player x has no units and instead create/give them for/to player y.

You can even give units to empty player slots, but it must be a specific player, e.g. player 1, not force 1. The units will be hostile and mostly do nothing though. Not sure if you can give orders. Whatever you do, trigger owner always must be an active player.

That's good to know. I'd like to know if they respond to AI scripts/move orders/Set Player X to Ally, but if nobody else is sure, then I'll try and do some experimenting this weekend and will update this thread in case anyone else comes along with the same question.