[O.B.Left]

Are they really on the elbow plane or appearing to be so because they drop their heads/torso (Tiger) on the downswing?

Hmmm. I'd say the player who establishes his Impact Alignments at Fix (his RFFW on the Shaft Plane) makes the adjustments required that you refer to above, bobs, waist bend, knee bend, skull line etc prior to Startup. Players who address the ball with their Right Elbows above the Shaft Plane must make these adjustments during the swing and suffer from the inconsistencies that result.

But to answer your question directly, Id say they are only on an Elbow Plane if their Elbow is on the Shaft Plane. Most pros do bob to achieve this impact alignment having addressed the ball with their right elbow above the Shaft Plane, Lie Angle.