Will there ever be a multiplayer or co-op mode?
Probably not. EDuke32 all but dropped multiplayer support a long time ago (there is a fork, NetDuke32, that improves it a lot), consequentially the map design and scripting for AMC Squad has evolved to assume the game is played in single player mode. Adjusting this, even if Eduke32 itself gets proper multiplayer support again, would be a gargantuan task which we're not likely to do.