The DOJ is part of the executive - so it is fiction that it was ever apolitical. RFK was JFKs brother, do you think they weren't coordinating DOJ's investigations into political opponents? (e.g. Jimmy Hoffa)

Congress created the DOJ, It is their job to police it. They can defund or even eliminate it. That's the check on it.

> jail anybody attempting to direct the DOJ for personal or political reasons?

When that person is the president and the Supreme Court has said they are immune from prosecution, you need something else to be a check.

Yeah, but it seems those other protections would/could possibly be a coin toss (eg a successful defense in a trial) and quite costly even if they never get to that stage, and you need a bit more certainty than that. Otherwise help can only come from those willing to become martyrs