I'm not sure you will get very far, because all airlines will tell you that seat assignments are not guaranteed. There's also something not quite right with the story, because the US DM lines are staffed on Sundays (24x7 actually), as are the general information lines.

In most cases, the flights go under "airport control" once you have checked in, meaning you cannot change them online and you can only change them in the airport. This sounds like the cause of you being unable to change the seats. If the person paid for a choice seat and did not get it, then he/she would be entitled to a refund of the choice seat fee.

You can complain about the mechanical delay but I believe a 3-hour delayed arrival time is below the limit for compensation. It doesn't hurt to ask but I'd not expect much. The extra connection and longer travel time is unpleasant, for sure, but it's something that happens with air travel and generally the airlines do not compensate unless you arrive at your destination a bit later than what happened here.