For instance, we wanted to ride Dumbo this September, but when we got there, we couldn't see any CMs about, so just got in line. I should have listened to the little voice in my head, telling me this wasn't a mainstream ride, as we got all the way to the front, only to find a turnstile

.
If in doubt, consult the Guidebook; in this case, it would tell you to "Go to the ramp located on RIGHT". Even if you didn't see the ramp right away, it would clue you in to look around for one.
The Guidebook isn't really that complete, but keep repeating "The Guidebook is my Friend."
(Think of Bruce, the Shark in Where's Nemo?)
The other issue we've had to stand our ground on, is transfering to a rental chair for Pirates. The first time we rode this in September, they persuaded my to swap to one of their chairs. The footplates on those things are ridiculously high, and by the end of the line I was in such a bad flare that I barely noticed the ride go by. From then on (yes, we rode Pirates a lot this last trip), I just insisted to stay in my own chair through the line. Most of the time, this wasn't an issue (they even forgot to ask me once of twice), but I did have a couple of CMs try really hard to get me to transfer. Needless to say, I stood my ground, as I didn't want another flare on my holiday.
You do need to know what your needs are and advocate for yourself. Just be nice, but firm if you can't do what they are asking. But, they are asking because they don't know what you need; if people get mad at the CMs for asking or are not clear about their needs, needs won't be met.
When we were at POC recently, the greeter smiled and said "You want to come on my ride." I explained right away what DD needed and asked whether that would be available. She said it would and we had no problems. They are trying a new procedure for wheelchairs, which I'm sure helped.
But, they do want as much as possible for people to use the attraction wheelchairs because they don't have to transfer those to the exit. They can have another wheelchair at the exit that you can use and they won't leave anyone stranded waiting for their own wheelchair.
So, if people who are able to transfer to one of the attraction wheelchairs do, it helps free up staff to handle the wheelchairs of people who can't.
Maybe those who are turned down for a GAC should ask the CM at the desk to write them a quick note (complete with a number to call) saying that their
ECV or wheelchair entitles them to use the wheelchair entrance without presenting a GAC.
IMHO, that would just make things worse. The policy is that people with wheelchairs,
ECVs, canes, walkers are allowed into the accessible entrance. If the line is a Mainstream Line, that means the regular entrance is wheelchair accessible.
All the lines are wheelchair accessible, so there is no need for a GAC to use the wheelchair accessible lines. I think people with wheelchairs/ECV having GACs that say they can use the wheelchair line is part of the problem.
Let's say that you are Sally NewCM. You were oriented to your attraction and you were told that you are supposed to let people with wheelchairs go thru your line. But, the first 8 people with wheelchairs show you a GAC that says "May use wheelchair entrance" before they enter the line. When the 9th person shows up, what will you do?
Will you think, those other 8 people didn't need to show me a GAC to use their wheelchair in line. Or will you think, "I must have gotten it wrong. Those 8 people can't all have been doing it wrong, so I need to ask this 9th person for their GAC."
I think that is happening now. Some of the people without GACs are the "9th person".
SOME of the wheelchair accessible lines are not easily accessible for some people's personal wheelchairs, especially of they don't fold (POC is the only one I can think of that isn't). Having a GAC that says you can use the wheelchair accessible entrance will not make that line accessible to someone's personal wheelchair.
If someone needs to bring their wheelchair onto/into the boarding area to board, that is even possible, but unless you tell the CMs you need to do that, they assume you can manage without it. The CMs don't know and since most people don't need to bring their wheelchair right up to board, they will assume you don't.
We are pretty clear about what we need to the CMs and have had not problem - we bring her wheelchair right onto moving walkways and as much as possible on any ride, we bring the wheelchair right up to (even touching) the ride car.
It's important for people to look at the guidebook/maps/signs and see whether it shows an icon of someone transferring from an ECV to a wheelchair. If it does, you need to talk to a CM to see what it means to you. It might mean that you are in a park ECV and will need to transfer to a wheelchair to even enter a line. If you are in a line that requires a transfer from an ECV to a wheelchair, having a GAC that allows you to use the wheelchair accessible access will not allow you to use your ECV - if the symbol is there, there is something about that attraction that makes it not accessible. If that is the case and you can't walk, there is supposed to be a wheelchair in the area - you may have to wait for it, but they are supposed to provide one. Again, if you have a problem, show the Guidebook, which says "Must transfer to an available wheelchair." It's not on each page, but is on the symbol description page of each guidebook.
If you are in a line that shows the wheelchair symbol (which means the line/attraction is accessible to both wheelchairs and ECVs) and the CM won't let you use the wheelchair/ECV in line, the appropriate way to deal with it is to get out the park map/guidebook and show the CM. If they still won't budge, ask for the supervisor. That will have 2 effects - it will get you the correct access and it will educate the CM who didn't know what to do.
Even though we have been traveling to WDW for almost 20 yrs with DD in a wheelchair (she was 2 when we started), we still get a new guidebook for each park each time just for those situtations.