Regarding Saintly reputation for Master Light Magic...
One tactic is to wait until you have Lloyd's Beacon, then queue up a bunch of completed quests but DON'T turn the quests in for the reward quite yet (these need to be major plot-related quests).
Set Lloyd's Beacon in front of the Master Light Magic trainer and in front of the places where you turn in the quests. Then recall back to each of the quest givers, turn in the quest, and finally recall back to the Master Light Magic Trainer.
That should boost your reputation enough to get it. And by avoiding the passage of time (by using Lloyd's Beacon) your reputation won't deteriorate down from Saintly (as it would if you traveled by horse, etc).
Sorry, but how fast do you guys' rep deteriorate?! It has never been a problem for me to keep my rep up (might be due to regular donations at temples).
Also, I think Angelic is the highest rep level?
As for plot related quests: those would be mainly council quests. I think you also get a boost from promotion quests, but I'm not sure. Either way, I suggest you start donating regularly once you start completing council quests and it really shouldn't be a problem.