I think Trump can’t fire Sessions. He was part of a backroom deal with someone. McConnell won’t schedule confirmation,hearings on a replacement.
Confirmations don’t go through the Senate Majority Leader. They go directly to the Senate committee that oversees the department involved.
The DOJ is under Sen. Grassley’s Judiciary Committee. Grassley warned Pres. Trump about not replacing Sessions shortly after confirmation hearings because Grassley felt that either the President or Sessions or both were playing some kind of game. It’s more than a year later now, and Sen. Grassley does understand things change.
In any case, the President can certainly use Interim appointments to replace Sessions and Wray without having to rely on the default successor from the bureaucracy.
I believe Sessions probably cut a deal to recuse himself on anything connected to Russia and Clinton/election matters in exchange for being confirmed. He probably also agreed to recommend that Trump keep Rosenstein and Huber.
I also believe that Trump is building congressional support so that he can finally clean out the FBI and DOJ.