The main issue here is that all these IC assumptions seem to come either from IC gathered facts (for example, EM files or the old Stillwater) and from OOC/IC confusions (like when after repeating the word ANGEL everyday on the OOC channel people start to take it as if their characters have always known that).
Anyway, even if I am not sure if Leo really wants people to retcon that fact, I do not see the situation broken beyond any hope. Let's just see what premise we have :
Old Stillwater was obviously angel or not hiding their affiliation. Other IC clues filtered out like with the EM files, etc. Leo was in Naraka before too iirc ?
1) What do we learn of that ? There is a high probability that Leo is still angel, and Stillwater too. It is up to him/them to prove it is not the case anymore.
2) Does that mean that they still are Angel ? No, period.
3) What does it implies if some characters still strongly believe it is the case and state it as a known fact ? They believe what they want, and let them believe it. Do you usually believe and trust a turnclaok completely ? No, there is a certain amount of time before trusting him. Here it is the same more or less, even if Stillwater and Leo have really changed, people will need time to believe it (even if OOCly we know they still havent changed their affiliations).
4) What kind of counter arguments can Leo/Stillwater's supporter find ? Well, rational thinking, I think, to begin with. "What makes you think that Stillwater is still Angel since the change of directors even if we clearly stated we are not (then link the relevant PR piece of info) ? What makes you think Leo is still Angel ? Never heard of capsuleers changing loyalties ?" It is expected for a lot of people not to believe it as per 3), but it also asks them to bring proofs that what you say here is false, and they just can't (unless I have missed some clear evidence somewhere). The very moment they will prove unable to bring proofs instead of slander, they are screwed rhetorically (make sure to use the word slander then).
5) What makes it interesting ? We have a corporation with affiliations that are more or less screaming angel for the public opinion, but nobody can really prove anything. It reminds me the mafia on some points. Everyone knows that guy is the Godfather, or that place is infiltrated by mafia guys, but nobody can do anything because they lack proofs.
That's mostly how I see it.
Also, my advice for Stillwater : find a good affiliation for your "public face". Build and be prolific around that "public face", be it amarrian holdings, or whatever.