The way I see it you(Allison's Aunt) have 2 hurdles: 1) Can you get the dining plan added with the discount and 2) Can you add the dining plan when one of the people in the room is not buying MYW/not on the plan.
Well for no. 2 the one solution may be to not include that person on the reservation... book the room for 2 with MYW. The big question there would be can you book a room with the AP disount but tell them you need tickets anyway.
That would give you 2 rooms, 1 room with 3 people (2 on the plan (one of whom owns an AP) and 1 person "snuck in" who will use that persons unneeded MYW tickets) and 1 room with 4 people with MYW. That would leave you with 6 people booked all on the same plan (again IF you can book a room with an AP and buy tickets for that person anyway) OK I know it sounds confusing and none of it matters if you can't do it, AND none of it matters if... Back to no. 1 - If you can't add dining with the discount. But if part 2 works you'd be half way there. You'd have 6 people all booked with the same plan (Hopefully with the AP room disount) Then you add the dining option to all 6. (Even though you have 7) Trust me, 6 people on the dining plan will feed 7 (except buffets) but 6 people at a sit down, when each one gets an appetizer, salad, dinner, side, dessert and drink, will feed 7 with no problem.
I know you're bending the rules a little for the 7th person, the other option would be to book the room for 3 and before you leave add the non exp. option to the one unused MYW ticket.
But using the other option you would not have to buy that ticket or the non exp option and you'd "save" $35/night on the dining plan.