So the defense argument to the claim that he committed high crimes and misdemeanors is basically "Nuh uh!! And you can't bar him from office cause he ain't the Pres no more."
One thing that I did not see that I was expecting was an argument that the actions he is alleged to have committed are not high crimes or misdemeanors because they were not dependent on his holding office.
In this context a "high crime" does not mean something like a felony and a "misdemeanor" does not mean what we usually mean when we talk about a crime lower than a felony. A "high crime" means something like a crime committed by virtue of holding a high office. It's something like corruption or abuse of power. It has to be related to the office. For example, they tried to impeach Nixon over unpaid taxes but decided that is a personal matter not a "high crime".
A misdemeanor means something like the original meaning: "mis" meaning bad or wrong and "demeanor" meaning outward behavior. But that doesn't mean bad personal behavior or conduct unbecoming a President. It means something more like "maladministration" which was a term that had previously been considered to be used but was rejected because that term could be construed to mean that Congress simply doesn't like the way a President is administer the office. Misdemeanors here means wrong or improper actions in carrying out the duty of the office. People have been impeached for things like drunkenness, but only because that affects their abilities to carry out their duties.
Of course the phrase "high crimes and misdemeanors" doesn't really exactly mean what it's constituent parts mean. The phrase at the time was a term of art that had come to have its own particular meaning that isn't precisely articulated.
It could be argued that the alleged actions by Trump are not high crimes or misdemeanors because they are not dependent on his holding office. For example, imagine if he had lost the election in 2016 and did the same thing he did this year resulting in a mob storming the Capitol. He could not have been impeached and barred from running in 2020 because he wasn't President or Vice President or holding any other federal office. Why would the action of a person not subject to being barred from holding office be able to be barred for exactly the same actions simply because they happened to be in office at the time?
For a "jury" of Senators who are mostly lawyers, that argument may have some resonance. But it might be risky because the House "prosecution" would jump on the fact that this was a betrayal to the country and Constitution, which would be a violation of the oath of office, which would certainly seem fall within "high crimes and misdemeanors".
But it does seem like an argument that should be considered, especially considering that removal from office is not an option and this is only about barring from holding future office, which means that Congress would be potentially overriding the free will of the people to reelect him, if they should choose, knowing his past actions.