The Constitution provides impeachment as the only remedy for high crimes, including treason, and misdemeanors committed by a sitting president.

Sitting presidents cannot be indicted and tried in the criminal courts. That is the current policy of the Department of Justice.

A president has constitutional power to pardon federal criminal conduct, that power may extend to the president pardoning himself, and the pardon power is preemptive.

Combined, those authorities give sitting presidents immunity from prosecution in the criminal courts.

I have a question: Do not those authorities presume that the president came to the office through non-criminal means? 

There was an attack by a hostile foreign power on the former United States. That attack was intended to and did result in Donald Trump being installed in the office of president. He now has the Constitution that he violated to get the office to protect him from removal from office.

If Mike Pence murdered Donald Trump, thereby ascending to the presidency, Pence could then pardon himself.

There is a convention of legal interpretation that the reviewing authority should not interpret the law in a way that leads to an absurd result. The actual case of Trump and the hypothetical case of Pence are absurd results and absurd in the same way: a person coming to the presidency illegitimately, criminally, and then using constitutional protections afforded legitimate presidents to maintain his illegitimate hold on the office.