Yup, the campaigns invest a lot in investigating each other.  (They also hire people to research themselves, to figure out what the opposition is likely to uncover.)

Yup, you can bet the Mueller investigation has looked at what they uncovered about each other.

Nope, this has nothing to do with whether the investigation is legit.

Odds are overwhelming that this investigation is going to find nothing even vaguely impeachable, but that both campaigns are going to be outed as doing some truly heinous stuff.  And that outing will be good for America, going forward.  

Liberals are nuts if they think this is going to reverse the election of 2016.  But it is time to uncover the sleaze that has become SOP in these elections, at least if we want to have a republic going forward.  Otherwise, both sides are going to quit accepting election results, and the country is going to self destruct.