I don’t think it takes two weeks to prepare to say, “Sorry, you bunch of morons, but Donald Trump is no longer the President and you cannot impeach a private citizen. Please read the United States Constitution before making further jackasses of yourselves.”
LArticle I, Section 3, Clause 7 covers:
1. removal from office (if still in office)
2. bar from holding future office (whether they’re being removed from office or have already left it)
In the event that an impeached individual is found guilty, (2) applies.
I think there is already more than one precedent of an impeached individual resigning or bring expelled before the impeachment.
Blount
Judge Mark H. Delahay
William W. Belknap
The defense in the Belknap case was:
“... The judgment which the Constitution requires you to pronounce is a judgment of removal and disqualification—not removal or disqualification.”
Convention rather than precedent suggests that Trump shouldn’t be impeached after leaving office.
But between the Belknap case and the Former President’s Act, the judgement of removal could be to rescind the privileged status of an ex-POTUS, and then disqualification could be to prevent a convicted person running for high office.
Since that is consistent with the letter of the law, it follows that “you can’t impeach someone who’s already left the position” ignores precedent and ignores the status conferred on ex presidents.