Surely there is a basic flaw in the argument that Trump cannot be impeached because he is no longer in office.

He has already been impeached a few days ago, while he was still president. What the Senate is doing is not to impeach him, but to determine whether that should take effect, and what that effect should be. That is a different thing.

Of course arguments about what is right or wrong, or what happened, are completely irrelevant. Nor does anyone expect them to feel bound by the solemn oath they have taken - "“I solemnly swear (or affirm) that in all things appertaining to the trial of ____, now pending, I will do impartial justice according to the Constitution and laws, so help me God.” Those voting to acquit will be guided by politics, and personal advantage. "Impartial justice" has nothing to do with it.