I know with free dining you have to purchase resort/tickets but what about when you pay for each person, can you buy tickets somewhere else and get a discounted room rate?
Pretty much the same rules that applied to the free dining plan apply to dining plan in general. It has to be part of a plan with tickets and rooms on the same package.
The only difference is that the DDP is also offered to people staying in DVC units using DVC member-points.
And as posted elsewhere, you can get a AAA discount on the room but you have to book a package so you have to do this through AAA. For some reason Disney reservations won't do AAA discount when booking a package, only for room only reservations.