I accidentally went to the khor minos outskirts before getting the armor to turn brint into brienne, and the save before I did that is quite a while ago. Is there a way I can edit my save file to fix that so I can get brienne instead?
Yes.
1. Go back to Hawkethorne (or in general far away from Undermountain and Khor'Minos).
2. Save to file and delete this flag MAD_COW_KILL from your savefile.
3. Boot up and go to the proper tile in Harvest Valley with Brint to start the quest.
This assumes you never touched the quest. Ofc if you did something with Brint in Khor'Minos it will be in save file and now or in future some random Brint can appear (depends on coding). Also could be that some flags are used for both Brint and Brienne so you will not see the same event again for Brienne.
Above I wrote from memory - if you still have problems I would need to look into your save file.