we are presented with an ending that acts in a way the whole rest of the book hasn't

I don't think that's so. Ogion's dying vision is of Ged healing the wound in the world. Every positive use of power in the series to date is brought into the frame by that vision -- I mean it would be there anyway. It's at best complicated and at worst problematic to think of the abuse Therru suffers as only an effect of the world being broken at the time, though that's how the villagers think of it, if I'm remembering right -- "this wouldn't have happened in the better days". The extent to which Roke has been presiding over a form of fear-driven patriarchal feudalism is more explicitly dealt with in "The Finder" and "Dragonfly". But in some sense, this is a world where magic can be used to uphold a benign natural balance, and that idea is what Ged spent his power for. It's not that the world is inherently just. It's that it has an inherent capacity for good order, which men have been variously perverting for a long time now.

All of which is an attempt at saying, I think "magic is only bad in this book until suddenly it's not" is an over-simplification.