I think you can just call it a spinner nut. Some might call it a spinner adapter, but that's what I usually call the little rings that center the backplate over the prop shaft.

I fly my SSE on a Futaba 6YG, which has dual rates but no expo. The plane is really stable at low speeds, and I'm not sure you need it. Mine just floats in like it's on rails. I might tap the rudder once or twice. No roll coupling, so the wings are rock solid.

Oddly, I've got about 30 flights on my SSE and I've never used the high rates. Never even tried them. The plane is so quick and responsive on low rates (rolls like you won't believe), I can't see much use yet for the highs.