1. There is no 2005 tax case here. The statute of limitations for such a charge has long since expired.
2. Manafort has made a career out of peddling influence in Washington to foreign governments. I think you'd have a hard time finding a Federal investigation involving Manafort that did NOT come under Mueller's jurisdiction.
That depends on how one construes Mueller's subject matter jurisdiction. Ellis seems to be of a mind that if it isn't Trump campaign related (what Sessions recused from), then it's not in Mueller's subject matter scope.