There is a great chance that none of this makes sense, because I never read fully through the write up, and don't remember the exact timeline, but could it be that...
Fitzy was innocent, and him and Cubs did the sanitizer thing like they said, but on a night that Fitzy didn't have it, he was infected. It was passed back to him but he was then mafia, so it is no longer around. He then lied that he passed it to CoKane (which we know is true) to "test" him, but really it's because it was destroyed when passed back to him.
Don't remember exactly when, but he was "surprised" he was still around after one night. Could that have been the night he was infected? Honestly don't know.
When we thought we narrowed down the AC to SCNCY and Cleaver, I think he pushed voting for Cleaver, knowing that SCNCY was the AC and trying to keep him around.