Re: why is Peter Strzok still employed & working at the FBI
I think Peter Schweikart had a pretty good explanation:
https://spectator.org/the-biggest-demonstration-in-dc-has-nothing-to-do-with-guns/
With Peter Strzok occupied in Human Resources at the FBI, it would be easier to keep a watchful eye on him. Although it's tempting to think of him as a double agent, the major problem with that idea is that he could be said to be engaged in entrapment of these other people. So I don't think that's the case.
More likely, he was kept because he's made a deal to testify without reservation in return for POTUS' word on a commutation, + keep his existing pay & retirement. Odds are that, given what he knows, it was a good deal to make–he's ultimately a minor player that could result in HRC, Obama, James Comey, + more going down.