Of all of the Twirl variants the MonoTwirl is my favorite. All of your creations look really good, but the extra measure of simplicity of the MonoTwirl makes it stand out nicely for me.
It also lives up to the slogan in your avatar quite well!
I would love to know why the left roll problem occured. I wonder if at the lower rotor rpm, there was significant ASL; and being a semi rigid rotor, maybe there was large enough component of the resulting moment in the lateral direction to cause this roll???