Which parts are you struggling with? tell me the story as it is in your mind and I'll give you some questions to send you in the right direction
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?