1. I would advise that when you first get Quickenings, you save your game by some mobs and try to put together some chains. Running out of MP isn't a problem this way, because it is easy to refresh on the save crystal. Quickening chains are important, so it is good to get used to them before that big boss battle.

2. I have only two characters who are totally dependant on MP, and the rest are hybrids. Sometimes I will start a battle with all my melee guys out to use their Quickenings, then sub in a healer and get to business as usual. Point is, if you give your party well-defined roles, you can have your tanks and dps blow their mp without worrying too much, because your casters are in reserve.