To respond to the "the seatmate didn't see this" question, given that BWI police plus the gate agent was involved, do you think the whole discussion took place on the aircraft?

Though it hasn't been stated yet, I suspect most of the confrontation went on in the boarding area.

The plane was ready to depart, and was only waiting for connecting pax. So it's a reasonable inference that this witness was already seated, and he wouldn't have been allowed to leave the aircraft while the crew is investigating the SS agent. So did this "seatmate witness" see the whole thing, or did he only see the parts on the aircraft??

More information is needed, but I think the key witness should be the BWI police officer. He's an impartial observer, having no conneciton to AA, and he has been named to have been present to observe the key offense, being abusive and threatening toward the crew.