If the elbow remains in place, then the pivot of the motion is the elbow, not the shoulder. So yes, if I understand you correctly, that would be driven by the elbow, not the shoulder.
I see what you mean, but I don't think that "driven" is the right word for it. You're definitely using the elbow as a pivot, but the motion is caused by the shoulder cuff muscles, and your entire arm moves during the motion.
But the upper arm just rotates in that case, it doesn't move (translate).
The way I see it both the arm+forearm need to participate in the arc of motion. The purpose is to get the longest radius to get the flatest arc possible, which makes drawing straight lines easier (since there's less arc curvature to compensate for).
When it comes to technical drawing, there's no one better than Scott Robertson. I regularly use this book as a reference when eyeballing my perspective just won't cut it anymore. Need to figure out exactly how to rotate an object in 3D space? How to project a shape in perspective? Look no further.
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