[Frostmourne27]

Re: Buatha
The chosen amount of magic isn't the max level. It can be increased in game by empowerment or by pathboosting items. However, your blessing, the effects of which can be seen at the bottom of the magic page, is based on what you started with.
I'm not sure I understand exaclty what you mean by the rest of your post, but I'll try to answer as best I can. If you have a god with astral three, he can cast ANY spell that requires astral 2, 3, or 4. However, he has to research those spells first. So, at the begining of the game, he can cast twist fate. If you research alteration 1, he can cast personal luck, an astral 1 spell. If you research alteration 9, he can cast any alteration spell that requires astral three or less: Twist Fate, Personal Luck, Body Ethereal, Luck, Baleful Star, Control, and Battle Fortune. His astral magic doesn't go up as you research more spells. If you want him to cast wish (alteration 9, requires nine astral) you would need to empower him, or give him astral boosting items.
Research works as follows: you need X points to get level one (starting settings can change what X is) once you put X points into that school of magic, you have all level one spells researched, for all mages. You then need to put a certain amount of research into that same school to get level two spells. (It allways cost more to research a level than it cost to research the previous level.) The manual, on pages 83 through 91, has a lot more detail about this.