If I use Shadowkeeper to change Edwin's specialty school, will it cause any problems? Will the opposition school change properly?

I don't see any reason why it would cause problems- although some banned spells may possibly appear in his spellbook. The opposition school is tied to the mage's speciality- not the mage himself, so there should not be an issue there.

Be sure to delete the newly banned school spells already in Edwin's book. If you make him a wild mage (which is perhaps the least suitable kit for him) be sure to add wild mage only spells to his book via SK. Also, some mage specialists have different min ability scores. Swap his CON via that required stat and it will be seamless. Although now I am thinking I can not picture him as a charismatic and romantic enchanter either. He is just evil and corrupted to the core in a disturbing and funny way.

Actually the stat requirements only affect if you can give the mage that specialisation to begin with - if you change the specialisation later not having the required stats makes no difference. Some BioWare NPCs break stat requirements for classes anyway (Minsc springs to mind). I would still take out the banned spells from his spellbook though, as keeping them there feels a bit like cheating.

Hm-hm, I know that, but for a perfect change it could still be doable...