Just got back this morning from the RIU NEGRIL.
I will try my best answer your questions about SHADOWS, the building is open air concept, serves as the lunch buffet during the day, and the Steak house at night. As you know the dining area is two levels, with three different sets of stairs leading upstairs, no wheelchair access. I did eat there one night and they only section a small corner in the lower area of it for the "steak house" so if you did have it on the main floor it would still be quite private, aside from the fact that everyone would be sharing the salad bar buffet.
I did see a reception on the beach, held in a tent. Looked really nice but I know you have to pay extra for this.
I also have a copy of the resort map. Basically there are four U-Shaped buildings for lodging, #4 is furthest from the lobby but honestly takes about 5 minutes to walk to, The lobby is in the middle, pools located directly in front (1 pool has a swim up bar, the other a volley ball net). The other end of the resort has the breakfast/dinner buffet and the two other a la carte restaurants.
I will scan the resort map if you would like.