I would like to model a helix curve that follows a paraboloid (shape of a reflector).
I tried to make a helix, but only arrive at making a linearly varying helix (following a cone).
The point to choose is the point where you want to locate the moment. A moment is always defined in relation to a point. I do not understand what is your problem. Can you describe your problem in more detail ?
You are really mixing up everything. 2D models are supposed to be applied when the loads and deformations are merely working in the plane itself.
2D plane stress is used for very thin parts: the stress components orthogonal to the deformation plane are zero. The orthogonal strains aren't.
2D...