I don't see a problem with the aoe toggle, and as far as buttons, that same button is used to begin BRs and for emotes outside of combat, so it's not simply a toggle for one thing.
as for the ease of aoes I think that cost is a bit silly since its the same as non aoe. as for strategy, if your using aoes you have to be more aware of your surroundings so that you don't put your party at risk.

The biggest problem at the moment on the curing side of things is the horrid targeting system, we can't even sub-target party members with simple up and down....
As for making -ga spells I don't like this idea at all, it was terrible in 11 when manually selecting spells as it simply cluttered the list.
As for enmity I could see an increase for it on aoes even if you only attack a single target or heal a single party member.
Over all it would require players to be more strategic in it's use and possibly more conservative, though mp regeneration makes short battles a joke for mages now