DVC points/cash room and the ddp?

[Raenstoirm]

Ok, this is confusing yet simple and I am not sure if I should post in here or under DVC. We are staying at beach on points for two nights (so this is one ressie). The 3rd night was only available for cash so we booked that (so a second ressie). They are linked so we shouldn't have to change rooms (or so they claim). Anyway, that link is what I am concerned about. Can we buy the ddp just for the points nights and not get it messed up on that checkin/check out day when we switch to the cash night?
The Dxddp plans seems to save us $150 (if we actually want all the food), but this trip is a 4 way split stay so trying it on a segment or two seems like a good test.