Can support be removed from say Carthage (in the 3 and 4 player games) if the Carthaginian army is defeated?
Yep. A good way for the Greek player to clean up Sicily and/or take control of Lilybaeum.
And while I'm at it, does the Grand Coalition card have any effect on the Gallic Events table in the 3 player game? I couldn't find anything about it in the rules but maybe I just missed it. I mean, if the Gauls are now the allies of the Etruscans it would make sense that they would no longer be raiding their new buddies, no?
In the playbook under Three Players - "The Etruscan player is immune to all Gallic event rolls the turn Grand Coalition (#36) is active."