My DD also has a cognitive disability. We went to
Disneyland in January and used the DAS along with FastPasses. We were very successful and I felt it was better than the GAC. My DD is a planner and likes to know what she is doing next and when so this worked out perfect for her. She even stated such when she requested a DAS card.
We came up with a situation that it was just not possible using the DAS and FP wasn't an option. My DD tried 3 times to solve this situation and she was beginning to get frustrated. I told her it wasn't worth it because if she became too frustrated and had a meltdown her day in the park was over and she was back at the hotel. She decided that she would try GR and see if they had a solution to her problem that she hadn't thought about. She also wanted to tell them that, in this situation, the DAS wasn't working. The CM who issued the DAS card to her told her that if she had any problems she could come back and tell them. My DD approached the CM in GR and told them she had a problem and explained to them what she had tried. The CM was very attentive to my DD and explained a couple of things to her. They worked back and forth with the situation and finally the CM gave my DD XX with instructions on who to give it to and what to say. He mentioned that he would make contact for her. It worked great! My DD made sure to go directly back to GR to thank them and to tell them it worked. Note: XX was not some kind of FOTL pass, or FP.
In our case we had something specific - one thing that didn't work when everything else about the DAS and FP was working.
I think if one approaches GR with the idea that DAS and FP just isn't going to work at all, then I think Disney has a hard time working with you because the DAS and FP is what they offer. However, I think that, like us, you approach GR with a specific situation where you tried and experienced difficulty and are open minded to solutions, Disney CM's can work individually with you. In other words, I don't believe that Disney is going to alter their entire DAS for you, all day, for every attraction, for your length of stay.
I don't believe that Disney would implement the DAS if they didn't believe that it would be successful 99.9% of the time. That is just my opinion because I certainly wouldn't implement something unless I was pretty sure of it's success.
Note: the use of "you" is a general "you" and not directed at a specific person.