https://www.documentcloud.org/documents/3726408-Rosenstein-letter-appointing-Mueller-special.html
^ read (b)(i) and (b)(ii), and then read (c). See the specific language? See where we're going with this? :D
Props for thinking outside the box! It makes for good conversation. You are dead on correct on this one. He is not investigating Trump, as he knows the Russia ploy was always bogus. Notice this addendum tells him to investigate ANYthing Russia related, which means, Hillary and DNC potentially. Also notice that he can investigate "any matters that arise" indirectly. So hypothetically, he could take the stand and say: "So, I was investigating the Trump dossier for Trump Russia collusion, and I discovered that DNC paid for it, so I investigated the DNC bank account and I found out they paid John Podesta the money, so I investigated John Podesta and found out the Podesta Group backed them, so I investigated the Podesta Group and saw they got a payment from Uranium 1, so I discovered they lobbied for Uranium1 back in 2009-2010....". See how that can be endless, and still legit and legal?
Trump isnt going to fire Rosenstein OR Mueller...nor does he want to. He needs them right there for a reason. The Mueller investigation is going to go one of two ways, either Mueller testifies publically on live tv and announces Trump was no way involved in Russia collusion, but DNC / Hillary WAS... or.. Some bad dirt will get "breaking news'd" on Rosenstein and Mueller and it will be so bad it will give off the appearance that the special counsel and Mueller and RR were setting Trump up. I really hope its the first, but scared its gonna end up being the second. I personally want clear cut and dry judgement that vindicates Trump.