In my small and not very good at it way - I'd have to say all, and also none of the above.
I'd say it comes very slowly, and while the finer points of action height, strings, EQ settings etc, can help - once you get a bit of a feel for the playing, you can probably do it on any rig...
I think the challenge is feel and control with the thumb as well as you can probably already control the fingers... that and teaching a finger to hook under and pull...
And (again, in my small experience, FWIW), Forget Me Nots isn't the easiest song to get right for a beginner. π