Welcome to the club. Different people will have different positions on your questions, but here are my two cents:
1. I will not do the dining plan while on WW. Even if it were free dining, I'd do a room discount (I have an AP). Although you can say you don't have to eat desserts, it's just too tempting. If I have to separately buy a dessert, I don't get one unless I really want it. Having it "part of the price" hurts me.
2. There are healthier options everywhere. I particularly like Sunshine Seasons at Epcot. There are carts selling fresh fruits and veggies now. That didn't use to be the case. You can also request carrots or apple slices instead of fries at CS restaurants. The TS restaurants will also work with you to get healthier options -- you just have to ask. We also order breakfast stuff and snacks from
www.gardengrocer.com.
3. All the walking is great. I never count it as Activity Points though.
4. Last April, before WW, we went to WDW and had the QSDP. I'm sure I gained weight, but when I saw photos from that trip, I immediately joined WW (mid-April). In August, while on WW, we visited WDW for a week and we did not do a
DDP, we ate at a wide variety of places -- including Yachtsman Steakhouse -- and I had a couple of Mickey Bars, a Dole Whip, and other treats. I also exercised at the BWV gym a few times. I kinda kept track of points on my Blackberry, but I was not Type-A about it. Upon return, I lost 2 pounds since the prior WI two weeks before. If you read through the thread, you'll find some people gain and some lose. But, they always acknowledge why and how that happened.
You have some time to go before July. By then, you will have really formed a habit and making wise selections will be second nature. We head back to WDW on April 1 and I know I'm ready.