In my view, multiplayer is great as long as you don't mess up single player.
Combat system for single player should in my view be: -Real time with pause to conserve the feeling of playing a BG game. This mechanic is part of the essence of what makes BG BG.
Combat system for multiplayer should in my view be: -(Like before): Real time with pause and then game admin can allow or not other players to pause.
-(New): What about paced real time with pause? --Let's say a combat starts, game automatically pause for X amount of time (where X is configurable by the game admin). During this pause, each player could determine their next action. Once action determined, each player could either wait for X to get expired or press space bar to indicate they are ok with the choice they made (if both of them press space, then game resume before the end of X). This way, a strange pause initiated by one of the player will never occur since pause will be handled by game instead of player. ---We could also define Y resume amount time. This would determine how much time game will run before next pause. Maybe Y should be hardcoded by the game rather than configurable to make sure everybody feels a similar experience in combat (also, if Y is too long, then you are at risk of having a player do nothing until next pause which is undesirable)
-(Both): Game admin could either choose "realtime with pause" or "paced realtime with pause"