Just some random thinking:

Can the airline change any passenger's seat on a whim? Yes. Even the various elite seating privileges are subject to change by gate agents and/or flight attendents. Should they? No, but they can.

I'd be interested to know what "legalese" the CSA cited during your phone call. If it was the ADA, she's exceptionally badly trained, because the ADA doesn't apply. The Air Carrier Access Act, which does apply (to the airline, not necessarily to you) doesn't have anything in it about FORCING a passenger to sit in a particular seat, just about offering seat alternatives that make things easier for the passenger. Besides, everything in the ACAA dealing with seating is about people with mobility impairments. The only mention of hearing impairments is about conveying information, not about your location in the plane.

CSAs and gate agents are poorly trained on this stuff and probably aren't the right avenue of complaint. If you want to escalate, you should ask for the CRO (Complaint Resolution Officer) - every airline has to have one, and that person is supposed to be trained, with annual refreshers.

If it were me I probably wouldn't bother, and the next time something like that happened I also wouldn't bother to try to educate the gate agent in anyway, I'd just keep repeating (as many times as is necessary), "Thanks, my current assigned seat is fine" while hanging on to my boarding pass with a death grip.