The part of this story that makes no sense:
"The couple said the agent failed to do something many airlines do in this situation: ask for volunteers to switch seats."
Yeah, don't know about you, but I have never seen that happen before at the gate. I mean unreasonable expectations are no excuse to treat people badly, disability or not. However without all the facts it really seems like this could have escalated because they were asking for special treatment and not getting it. This happens all the time without disabilities involved. I feel like the agent could have rolled their eyes at any passenger for this request. Not that it would be ok to do that.