Wow! You really have a great plan there! The only thing I have to add is that if your DD's want more of Disney there are free things to do on property. On one of your down days (if you need more of a Disney fix) you could go to Fort Wilderness and go to the petting zoo. Other things are the Boardwalk at night (has some cute entertainment and good places to eat), Downtown Disney (you've already planned that), and resort hopping. If you have lunch at one of the monorail resorts, you can park there for three hours and use the monorail to tour the other two resorts. We have small children (3 1/2) so we never do more than four days at the parks. We know we'll be coming back several more times and any more than that would be overwhelming for them. I don't think you'll have that problem with your girls, but if they've had enough remember that the days never expire. Better to not use days than to use them and be miserable. Whatever happens, I'm sure you'll have a magical trip.
Oh, and BTW we've stayed onsite and offsite and a rented home. I love onsite, and will probably stay there next trip. However, the space and privacy that the rental home provided was so relaxing and enjoyable that I wouldn't hesitate to stay offsite again.