Huey N2 RPM
The Huey does have a roll / pitch couple - we used to demonstrate it in effects of controls. Pull up on the collective - nose goes up and you roll right. Push down, you roll left and the nose goes down. I always thought it was to do with the horizontal stab (g'day Helmet Fire) but have never worked out where the roll comes in. Perhaps it's phase lag?
As for the 64 / 66 question, the reason it was changed to 66 in Australian service was to reduce the risk of pilots losing T/R auth in the hover at high AUW after completing their cruise at 64 (g'day Nick).
There was an issue a while ago with N2 spur gear failure - a NZ operator crashed I believe, among others - where ops at 65 created a resonance which led to failure of the N2 spur gear. This led to loss of the OS governor and torquemeter boost pump, leading to indications of low Tq while the engine accelerated due to sensing low N2. End result - engine failure.