I can understand why you are frustrated. I have spoken to less than helpful people (in general) on call lines when I call them for their 'specialty' and it is frustrating to get the canned response, even though I know their job is to read it. The insurance company is the worst. This kind of response from them just makes you feel worse. Especially since you called and flat asked
I think you know they won't do anything. Probably won't change what they have available, but probably won't offer it anymore except in accessible rooms. If the seat was that unstable they probably got several complaints.
The moral of the story (which I'm sorry won't help you this past time) is that when you or your child have special needs you have to meet the needs yourself. Any company, hotel, restaurant, etc is only as helpful as the worker you are dealing with wants to be. If they are having a bad day, being poorly paid, just doesn't care, we are putting our health in their hands. Sadly you really can't depend on someone else for things you must have.