I’m not a constitutional lawyer and I don’t play one on TV.
But the argument that a former president cannot be impeached for acts committed while he was in office seems flawed to me.
Imagine the harm a president could attempt to cause in his last day or two in office if this were true. He would either enjoy presidential immunity or be protected from impeachment.
No president has tried to pardon himself because no one should be a judge in his own case. This fundamental rule of law is based on sound public policy. Similarly, a sound public policy of accountability demands that former presidents be held accountable for their acts, including the ones committed in the 11th hour.
Ethical leaders seek to do no harm. If they make a mistake, they admit to it. If they cause damage, they seek to repair it. If they deserve punishment, they accept the consequences.