The response from the airline looks a lot like a form letter. After the fact, that's about the best you're going to get.
Once the plane door closes, the GA is out of the picture. The FA is now the one to deal with questions such as seating. Flight was oversold, so FA had no option there. After the fact, no help here, either.
Sadly, the time to deal with it is BEFORE the door closes. Afterward, the best you will get is a form letter.