Choose one spell which you have the ability to cast. You can cast that spell spontaneously by sacrificing a prepared spell or spell slot of equal or higher level.
I suppose this might be an oversight, but would this allow you to cast an opposition school preferred spell to be cast using a single slot?
Definitely looks like an oversight to me. I'd expect variation from one GM to the next on this matter.
Looks legit. You have a knack for one thing in an area normally outside your comfort zone. It's a feat!
There is a feat that is cheaper in regards to getting rid of opposition school, but preferred spell is pretty nice. Especially combined with spell perfection. Though now you spent to feats on a single spell.
True, it's better if you were considering taking that feat anyway.