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[The Velveteen Ocelot]

3. This isn't a legal proceeding. It's a political one with its own rules.

The consequence of being convicted after being impeached is normally that you lose your job, not that you go to jail or have to pay a fine. In this case Trump is already out of office so the only consequence (and the reason they are still doing it) would be that he can't hold any federal elected office in the future. He has no exposure to legal penalties at all. The impeachment trial is not in any respect a trial as occurs in the legal system, and there is no rule that the senators can't act as "jurors" regardless of their personal knowledge of the basis of the impeachment. (A bit of trivia: In the medieval English common-law system, jurors acted as investigators and went out and collected facts - so the impartial jury that was fed all of the facts during the trial is a relatively modern invention.)