And I'm confident Brienne's not going to be a corruption path, because people really like Brienne and it'd be a massive bummer if it came out this late in the process that making Brienne was evil.
Well....I'm almost certain we'll never get the game outright telling you Brienne is a corruption path for the reasons you've stated.
But at the same time, as you've said, the neglectfulness and apathy that's needed for the transformation to even take place after all the events and Brint raising his concerns multiple times
should be ramping the Corruption meter up excessively, considering it's champs fault he is wearing that armor in the first place.
Throw in the option for the champ to force themselves on Brienne the moment the transformation that they've basically orchestrated themselves on her is done(at the high cost of a hefty 3 corruption), and that their personality itself seems to make a sudden inexplicable shift after the change, and congratulations, you've got a Corruption path in all but name.
The game might not tell them it, but yeah, their character is
kind of morally repugnant at this point if you're not headcanoning the entire process away and doing something like pretending Brienne is some new companion unrelated to Brint, who must have ran off or died in a tragic gym accident offscreen.