You read my mind, I have been thinking this thought since the Fall of 2017
If you remember, Trump, Sessions, Mueller & Rosenstein had an 11 hr meeting the day before Mueller was appointed SC.
Q also had a drop around that time about paraphrasing, "Once a Marine, always a Marine & who do the Marines answer to..POTUS.
Speculation back then was that, POTUS called Mueller back to active duty and this was his mission..
I sure hope this is right...,
I am re-watching a video about Rosenstein. I think you could be right. Everyone knew Mueller could not be head of the FBI again. Why was Barr and Sessions so friendly to Rod at his retirement. Hmmmm.....