Let me see if I can answer some of your questions:
I think a week wouldn't be too long at all. It'll give you the advantage of spreading out your park time, so you don't feel like you've got to rush-rush-rush every day. When my mother & I take DD in April she'll be 18 months and we'll spend an entire week. We're only planning on doing half-days at the parks (sometimes mornings, sometimes evenings) and spend the rest of each day swimming, napping, etc. We're going over to Fort Wilderness one day to "visit" the horses. We'll spend a morning walking around DTD. Especially if you're thinking about a Sea World visit (which I recommend, DD loved it at 1 year) I think a week would be the minimum time I'd stay.
Certainly MK will have the most rides/attractions for your DS. AK is always great with the animals. But I wouldn't totally discount the other parks. My DD loved Nemo & Spaceship Earth, she made it through Universe of Energy (a 45 minute ride!

) with very minimal fuss, and she really likes walking around the World Showcase. There's a lot of "street" entertainment in the various pavilions. At the Studios she enjoyed Voyage of the Little Mermaid and the Great Movie Ride. There's also the new Playhouse Disney show, and the new Toy Story ride should most definitely be up & running by October. I will say the Studios is our "maybe" park for our upcoming trip. If she seems to be doing well we might go for a couple of hours on the "off" day I have built into our schedule. The other three parks will get two half-days each.
I think your plans so far sound great. Taking a toddler to WDW can seem really over-whelming, but the key is to go slow, take a break each day, and stick to his eating/napping/bedtime schedule as much as possible. We haven't had any major meltdowns with DD yet (knock, knock, knock on wood). I would definitely recommend a parent's night out as well. I'm sure other folks can provide babysitting suggestions...I've been lucky enough to travel with the grandparents, so they've always taken her.
Have fun planning, I know you'll have a great trip!