You're on the right track! try thinking about the logistics of a potential impersonator, who, when, why etc. there may be a little more to the "blackmail", because why would gander need to be poisoned if he was meant to be the one investigating this blackmail in the first place?
I mean, if he was investigating the blackmail, presumably the blackmailer poisoned him to stop the investigation (and if that's not the correct answer, then I know so little I'm going to need a more detailed hint).
As for the potential impersonator, I think it's got to be Pearl or Lloyd, but I have no idea why either of them would want to do that. Maybe just to throw off investigations into the time of death?