Probably not. To get the DDP you must book a room and tickets through Disney. Places like Expedia can rent Disney rooms, but can not add the DDP.
The only people who can add the DDP without buying park tickets are people staying on DVC points, AP holders paying rack rate or using an AP discount, and some military deals, but even they have to book their rooms through Disney (or DVC services).
Depends. If it's a third party broker (like Expedia or Travelocity), then no.
The dining plan can only be added to a package reservation that is booked through Disney (or booked for you with Disney through a travel agent.). DVC members booking through member services are the only exception to the package requirement, as well as occasional booking opportunities given only to Disney cast members. Annual passholders and those eligible for the military discount can get a dining plan without buying tickets as part of the package, but they still have to book a package through Disney.
So it will depend on whether you found that deal through a third party broker or through a travel agent that books with Disney on your behalf. Most likely, it is a third party broker and ineligible for the dining plan.
Yes, you can buy the dining plan when you rent DVC points (you're not eligible for the free dining though, you'd need to pay for it). The owner you are renting from must add the dining plan to your rental reservation. Because DVC Member Services will not deal with anyone except the actual owner, you will need to make arrangements with the owner as to how you will pay them for the dining plan.
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