Yes, I would add her to your dining reservations now, especially if adding her bumps your party up to needing a larger table (e.g., from 4 people to 5 people, meaning you go from a 4-top to a 6-top) - otherwise if the restaurant gets booked up, you may find that there are no tables left for your party size if you try and add her later. If she dines with you but is not on the DDP, you can absolutely pay for her out of pocket and the rest with the DDP. It's no different from a "split check" situation in your typical restaurant -- just tell the server which meals are being paid-for with the DDP and which are being paid-for out of pocket, and they can charge you accordingly.
I think it's interesting that they said that if you added her to your room reservation too last-minute (less than 3 days out), your neice wouldn't be on the DDP with you, but would just be added to your reservation as non-DDP guest. I was always under the impression that if you wanted the DDP for anybody in the room, you had to buy it for everybody, which has led to consternation for some guests (e.g., having to buy the dining plan for all guests in the room, even if one person has severe dietary restrictions or just had laproscopic surgery and won't be able to get any use out of the DDP). If what they told you is true (and if I read your post correctly and your intention is that your neice be added to your room or room-and-ticket package reservation), you may have just discovered a loophole!