OK, instead of starting a new post every few hours, I'm going to continue asking MM6 questions in this one.
How is spell skill calculated when casting from scrolls? Based on caster skill? How is it calculated for knight then. If he has 0 skill, shouldn't damage spells (the ones which do x per skill point) do 0 damage?
I don't think the other answer was right about this.
If you notice, the scrolls have different "quality", the cheapest ones are discolored and brown, and the more expensive ones look newer and fancier. I believe this quality refers to whether the spell is cast at normal/expert/master level. I don't know the exact level the scrolls are cast at, honestly scrolls aren't too important in the game, except maybe a fly or waterwalk spell in a pinch.
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).
Just as an important aside. In general, Council and Promition quests all raise your rep substantially, _except_ the price fixing quest, which lowers it a lot. Either do this early on, and then donate back to respectable, or if you already have angelic or something I'd wait until you get light mastery. I don't think it is too much of a spoiler to tell you that after you get light mastery, you don't really have to worry about reputation again. (Though you can get put in jail in some situations if you have a super low reputation, but just donating will fix it easily)