Can you be impeached after you leave office?
I agree that aiding and abetting a traitor is itself treason. No argument there at all.
Impeachment after leaving office removes benefits and the ability to further hold public office, so it does have an important use.
The two-thirds (67) Senate vote requirement to convict is almost impossible to achieve in contemporary conditions.
Yes. There is precedent for impeachment proceedings after the person has left office.
It probably won't happen, but clearly granting clemency to domestic terrorists and military traitors is in itself treasonous.