For the DDP, you only have to buy a one day ticket but you do have to pay rack rate (or
AAA) for the room. The DDP is based on the number of days you STAY, not the number of ticket days you buy. The rack rate requirement may only be true for the "free" DDP offer, but I know I had to do it for our September trip. You can apply that one day ticket to your AP renewal. But it's my understanding you can only apply one "credit" per AP renewal. So you couldn't get three one day passes on three separate trips, and apply all three of them to pay off next years AP. You can apply three passes to three separate AP renewals (i.e. one pass per person, one credit per person). I hope this isn't too confusing.
Anyway, as far as the DDE discount, It's worked well for my family. I'm also a seasonal AP holder (and a Tampa resident, HOWDY neighbor!). I bought the DDE before our spring break trip and used it at: WCC, Boma, Sci Fi, 50s PTC, Trails' End, Kona, Tusker House, and AS Sports Food Court. They didn't take it at Teppanyaki, but I did get a 10% AP discount there (many of the TS restaurants give you that 10% AP). I actually did a little spreadsheet before the trip, and compared my savings to my cost. We are very light eaters, and usually split meals. My spreadsheet had us breaking even just on that trip, and I ended up doing several more food court meals than I had planned, so I actually saved something like $75 (over and above what the AP savings would have been) that trip. Since then, I've used it at Pop Century's food court twice on day trips.
Another benefit to the DDE is that you can get free valet parking at the resort when you go in to eat, and free theme park parking when you go into eat. Since the seasonal pass doesn't cover parking, I checked into how this would work. You would pay for the parking and go into the park. Use your DDE when you eat (obviously at a DDE restaurant), then go to guest services and show them your receipt for parking and your meal. I was told that it has to be within 3 hours of arrival. So far, I haven't actually done this, since we've eaten counter service on our day trips since I got DDE. One place it should work for us is going to AK. We typically eat at Tusker House for lunch, so I should be able to get my $9 back for parking there.
There's a very complete listing on all ears that shows all the different discounts available, which should be able to answer specific restaurant questions.
Hope this helps!