There is something to this, as I'm not sure impeachment would be available/necessary because if he is found to not be a "natural born Citizen" as the Constitution mandates, then he is a usurper holding the office of Commander in Chief in direct violation of the Constitution.
Impeachment is for a President, not a usurper of the office.
I do think we need to research this. I thought a governor in the upper mid west was removed after he was elected and was found to be ineligible. I also remember another similar case as well. Not sure of invoking the 20th at this point, that was the responsibility of Congress.
But we can’t accept impeachment, it means he was president and all his “accomplishments” stand.