There are many valid models for these pipe supports. While the most elegant, most complete model should perform well under all stiffness and load combinations, such a model is usually a waste of time. Simplifications are usually the answer.

A technician can model; an engineer knows why one is better than another.

To answer your questions:

1. Yes, the midpoint MAY be a valid location to start your dummy leg for the elbow.

2. No, the "relative anchor" (Node/CNode) is not required.