The other argument for this impeachment being unconstitutional is that the Chief Justice of the Supreme Court has refused to preside over the senate trial.
That’s what I’ve heard too. The fact that John Roberts isn’t there means it is not an impeachment trial. It’s a Senate hearing, a Senate session, it’s a Senate proceeding, but it’s not an impeachment trial of the president.
The constitution says that in a presidential impeachment, that the Chief Justice shall preside. It’s not optional; the Chief Justice must be there for this to be an actual official legally binding impeachment trial of a president.