Hey Miria! I'm going to jump in and answer based on what I think I've read from the forums, somewhere.
As far as I know, you can learn magic on your own through a book, an npc, or through another player who knows the specifics of that magic. Moderation is not required, but I don't think you're going to learn much unless you're being taught by an expert, whether it's an npc or a pc. And there are no npcs in Avanthal who have knowledge of magics that are seen as dangerous or destructive, so no voiding there. Anyone seen practicing these dangerous magics will be punished by the Icewatch. Also, I'm taking back what I said about reimancy in my earlier post, since I just realized it usually goes hand in hand with Ice Reaving after all.