RE: Is there a constant speed RC prop?
Not a true constant speed prop, but there is a variable pitch prop for electrics only. With it, pilots have performed reverse 3D maneuvers like hovering nose down.
There is some R&D going one with larger turbo prop models, I'm not certain if there is a production variable pitch prop available yet.
As for the middle of the road, glow engines, I haven't seen anything in that range.