There's nothing in the Constitution that gives the President immunity from prosecution, but the Supreme Court ruled that it is there.

As for gerrymandering... there are laws against gerrymandering, but the Supreme Court threw them out, except under conditions that are both vague and impossible to achieve. Unless the state legislature is literally twisting their mustaches and saying, "we are drawing a map to deny you representation" while tying a black person to train tracks the law has been invalidated.

And while you may personally agree with their decision the fact that the laws stood and were fine with decades of Supreme Court scrutiny shows that it is not a simple matter of states running their own elections.

As for the "no states having compact", I expect the Supreme Court to come up with some justification even if Congress allows it.