1. That was bothering me ever since I saw the episode for the first time! If only one drop of it can change him back, Donnie would of had more than two doses!
2. That wasn't retro-mutagen probably. Because Karai could change back except for her eyes and tongue and stuff.
3. This isn't a question. Plus my big sister thought the same thing. She thought he was going to make some of retro-mutagen, and throw them at the turtles to change them back to normal. But he said MUTANT! MUTANT I TELL YOU!!!