We used the DAS for my ds with autism this January. When you walk up to the turnstile, you'll ask the CM if you could please have the DAS for the ride. They'll say sure and ask for the magic band of the DAS person. The DAS person does not need to be there, only the band. When they swipe it, the CM will see all the names of people in your party that day, and you'll tell them who is going on the attraction. It's really easy peasy. Once or twice maybe we had a CM who was new and didn't know how, but usually it was a snap. Just swipe, they tap tap, and you're done. Mainly you just have to wait for the correct CM, because there will only be one CM who has the (I forget, the phone, tablet, whatever it was) to do it.
You are correct that tiers, etc. won't matter. You'll be able to get a return time for any ride. If the posted wait is less than 10 minutes, they'll shoo you right in to the FP line already, no waiting at all.
The thing is, if the FP line is long, it's still gonna be long. Like we wanted to do FEA, and I think we waited in line 45 minutes, even with the DAS. I just didn't pick the time very well, should have gone first thing. Same for 7DMT. Go at a low time AND use the DAS if it's really important to keep the lines down. Touring Plans' is really helpful.
It was a lot of work planning our trip, but the DAS worked out really well. MK is the worst to plan, because there are so many rides. The other parks also have shows. I was surprised by the waits for shows. Even if you have the DAS or a FP, there's still waiting. The Frozen Sing-Along had a wait to get in. You didn't get seated right away. There were tvs, but he had to stand with people and not be touching and bumping. Muppets 3D at MK, same gig. We got through the line and then had to wait for the next show. Turtle Talk with Crush also had a holding pen waiting area. You might want to have a game plan for those areas, like a special snack you brought just for that. I don't know what your disability is, just saying those holding pen waits were a surprise for us. So me, I would never want to do a regular standby que AND the holding pen with ds. That would be awful, basically a double wait.
We waited in one line at MK because my mother zoomed ahead with him. I thought oh, it's a carousel, it will load quickly. OH MY LANDS. Huge mistake. So don't under any circumstances let well-meaning relatives do that, kwim? If you need the DAS, just be draconian. Better than dealing with the consequences.
You'll be really glad you spent this time planning when you get there. I spent an absurd amount of time, but our trip with ds went really, really well. We're going back again, and this time it's a bit less hard just because I have my brain wrapped around it and know what happens. But definitely spend the time, even if it feels overwhelming.
Are you doing the mobile ordering for food? It's SLICK and they've expanded it. You really, really, really want to do mobile ordering. Oh my lands. There is no DAS for food. Actually, they magic'ed us, bless their souls. I tried to mobile order our BoG lunch, didn't get it done, and they, bless their souls, magically opened a line for us when we got up there. I was so tired and exhausted with him, and all of a sudden they are like here, come to this line we're opening!! For real. But don't plan on that. Personally, if you really, really have issues with waiting in line, I would do TS or pick QS that has mobile ordering. I think they just got it at the big QS restaurant in MK. We stood in line there and it was bad (tight, loud, overwhelming, fast). Food was great and we found a fast spot to eat, but I would never do that again without the mobile ordering. We're going to try the Satuli Canteen at AK when we go because they have the mobile ordering. When I have elderly relatives along with ds, they just can't handle his behaviors when waiting. If he gets overloaded, it spoils EVERYONE'S trip and makes EVERYONE stressed. So mobile order, DAS, think it all through ahead. We had a great trip and you will too.