I believe there is a gate across the entrance to the campground prohibiting entry when closed.

The Riana CG is gated, and I assume that you cannot. Contact the Abiquiu Project Office, 505-685-4371, Abiquiu@usace.army.mil.

You could dry camp below the dam in the BLM area. Call the COE visitor center (505-685-4371) and ask the rangers about the road below the dam-- it's often pretty rough and at its best it is not suitable for a large motorhome.

I don't believe so. The entry gates will be locked I think. The best way (or a way) to find out is go to recreation.gov and see if it will let you reserve the day(s) you want.