Also, the way I took it, the biggest risk is Zodiark fragment empowered voidsent + fighting it in the 13th = way too much dark aether. Y'shtola noted that their wards were starting to fail and Azdja basically got instantly converted by darkness once the protective ward on her was removed. Even WoL isn't immune from too much aether imbalance, although they have a much higher tolerance level. Plus they actually have to fight Zeromus in the 13th to even have a chance of saving Azdja, since the 13th doesn't allow death (she would just straight up be dead if they killed Zeromus in the Source).
Now as for how to reverse transformation due to aether imbalance, we haven't seen that. Azdja didn't seem totally transformed, just overwhelmed with dark aether. Tempering is due to that, but they established that the porxie method didn't work on things that had been fully transformed due to imbalance in aether (aka, cant turn a fullblown sin eater back into a person). I'm sure Y'Shtola will MacGuyver some solution, since her proposed plan seems to be somehow channeling light aether from the 1st into the 13th as a way to weaken Zeromus/the massive dark aether imbalance