On the matter of ordering an investigation to be dropped entirely, remember: Because of separation-of-powers principles, prosecutorial discretion is a basic feature of our criminal-justice system. Congress and the courts may not force the executive branch to pursue a case, no matter how serious the crime or compelling the evidence.
There are often good reasons for not pursuing a viable criminal case. Every day, throughout the country, the FBI forgoes various investigations and U.S. attorneys decline prosecutions. Sometimes bringing a criminal case would harm foreign relations or intelligence operations. Prosecuting a business that has committed a serious financial fraud may put hundreds of innocent people out of work. Sometimes a politically fraught prosecution might divide the country and feed a perception of politicized law enforcement (which, one can safely assume, is why Donald Trump soured on the hot campaign idea of prosecuting Hillary Clinton once he became President Trump). Sometimes, a person — particularly one with a record of patriotic service to the country — has been laid so low by scandal, professionally and personally, that prosecution would be overkill. (This was the case with Nixon. And, I suspect, it is President Trump’s inclination when it comes to General Flynn — note that the alleged Trump–Comey conversation about Flynn is said to have happened the day after Flynn’s ignominious resignation.)
Clearly, the fact that one may disagree, even vigorously, with an exercise of prosecutorial discretion does not make that exercise corrupt. To prove corruption, the abdication would have to be egregious and patent, because a president must have at least as much discretion to decline investigation and prosecution as the agents and prosecutors who are his subordinates. Those subordinates exercise extraordinarily broad discretion, which, as courts have repeatedly acknowledged, is judicially unreviewable.