Mueller was FBI director back then, I think. But right now, everyone's trying to cover their own arses, blame everybody else. Mueller's people say he and the Feeb had more than enough evidence to throw away the Epstein cell key but the prosecutors, including Acosta, tied their hands. (In fairness, it is not actually the FBI's job to prosecute people; that's what actual prosecutors are for). For his part, Acosta says "intelligence people" (Mueller's FBI? Comey's? Or people even more powerful than that? People from Arkansas perhaps?) ordered him to leave Epstein alone, so he did.
As for JE's money; well, they say he's a maths prodigy. So, add dodgy offshore fund-management, blackmailing the rich, famous and powerful and confidence trickstering to that and you're not going to be short of a few quid, are you. Although, I gather his actual wealth is probably over-estimated.