I'm going to throw sone other ideas into the mix. We have had very good luck using skyauction.com. There should be availability for the week you are looking at for a 2bdrm condo at Sheraton Vistana or some of the other top resorts. If you go with a Ready,Set, Go auction, you will probably pay around $250 (including all fees) for the week. Since you are planning on having lots of down time enjoying the resort, somewhere like Vistana, Orange Lake Country Club, or Cypress Point Grand Villa's would be ideal with their multiple pool areas. Also, since you will have the time, you could consider taking advantage of the time share presentations. You would usually get a couple of adult tickets for free to a theme park of your choice, and it takes about 1.5 - 2 hours of your time. That might be a good way to stretch your budget to include a third park visit.
I highly recommend attending Mickey's Very Merry Christmas Party as your MK day. The tickets to the party are less expensive than a one day ticket, and they will let you in as early as 4 pm. It goes until midnight, so if you can last that long, you would have a full 8 hours in the park. It is great to be able to see most attractions with little or no wait time. They also give out hot chocolate and cookies, and have a beautiful Christmas parade. The price includes a free family photo as well.
Also, if you've been considering giving Universal/ Island of Adventure a try they have a special on right now where you can buy an adult 2 day/2 park ticket and get a child's ticket for free. In addition, they also add another 2 days to the ticket so you get 4 days/ 2 parks, and a free child's ticket for about $100. We absolutely LOVE US/IOA.
Seaworld tickets also include a second visit free, as long as it's used within 7 days, so 2 days activities for the price of one visit.
Hope this helps, have a great vacation!