OP, I think you had good intentions posting here, I'm sure you understand this is a sensitive topic for many posters. I've only used a GAC on one trip (a few weeks ago), and I had all good interactions with CMs including the HM and PoTC. It sounds like what folks want most is to be heard, it can't hurt if you pass that along to your fellow CMs.
I think Guest Services also needs to do a better job when they give out the GAC of explaining what may be/may not be possible, so that the guest doesn't have expectations that can't be met. Perhaps a pamphlet that lists the attractions and what can be done for the most common issues - ex. separate wheelchair entrance, or no separate entrance available, separate waiting area available in shade/air conditioning or no separate waiting area available. Something so that folks know what to expect at each attraction, and can decide if they are able to work with that or not and need to skip it.