It shouldn’t matter what Mueller finds before Trump was POTUS, he can’t be impeached for actions taken before he was elected. Unless this ass is going to indict Trump. If they try to remove him, that’ll be when the real resistance starts.
Rosenstein made sure of it:
“(ii) any matters that arose or may arise directly from the investigation; and...
(c) If the Special Counsel believes it is necessary and appropriate, the Special Counsel is
authorized to prosecute federal crimes arising from the investigation of these matters.”
https://assets.documentcloud.org/documents/3726408/Rosenstein-letter-appointing-Mueller-special.pdf
Where is that rule written? If there are 218 votes they can impeach him for whatever they want - and will.