If one player drops out, can his role be taken up by the AI?
You can re-generate the script half way through a game, so yes. Similarly if another player wants to join, or maybe if one player gets destroyed early on, an AI faction can be taken over by a player.

If any of you are interested, UltraWar is starting a campaign.
https://forums.totalwar.org/vb/showthread.php?t=65101