Yes he is. But we need to consider the idea that Powell and perhaps Barr mishandled this.
Covington et al now has free rein to testify against Flynn. The also have motivation, as Powell opened the opportunity for them to defend themselves against accusations of professional malfeasance.
Not sure how this is going to play out...
Do they? Attorney client privilege remains after representation ends. It isn't absolute, but I don't see much of an opening here for Covington to testify against Flynn.