Yes, it is possible that he could activate on Sunday, or if the sample that sent him to the list was taken Monday - he could activate Saturday. Provided all other conditions are met (ie, no fever for 24 hours and symptoms gone or "improved").
Based on the updated policy, every Bills player on the list except Cam Lewis eligible to return as soon as tomorrow if they have not had a fever for 24 hours and have improved in their other symptoms sufficiently to participate.
Why? The rules changes apply going forward. He was fined for breaking the rules months ago. One of the new rules is wearing masks at all times indoors, so I don't see the league changing the fines for mask violations.
No, buried in there is a "test out" provision where vaccinated players can come back in less than 5 days with 2 negative tests. Of course the likely reason for becoming negative in less than 5 days is a false positive.
Yes, he has been off the list for a while. However when he got Covid, he was reportedly sick, so quarantines and 10 day rules were secondary if he was too ill to play.