The Constitution gives Congress SOLE power to launch and try impeachment (U.S. const, art. I, sec. 6).
Show me where the Constitution authorizes the President to appoint a special investigator to investigate the President. It is not there. It is senseless as well as unconstitutional. The whole thing stinks and needs to be shut down.
Read that again and think about the implication of what you've written.
Mueller is not investigating President Trump.