Ringer:
You are correct when you say getting the right forearm on plane deals with the right shoulder and right elbow. Through Homer's research, he found that the hands/arms tend to follow the path of the shoulder (so long as we do nothing to alter the path of the hands). What I mean is, from the top, if we thrust our right shoulder towards the inside aft quandrant of the ball, our arms/hands begin to follow. To ensure our right forearm/hands are on-plane, we must ensure that are elbow is on-plane. This is the reason Homer wanted us to go to impact fix....it shows the proper alignments of the right forearm, bend in right elbow, etc. As Homer stated, the as the right elbow gets on plane, so does the right forearm. In many people's swings, the right elbow may only be on plane during the iniital takeaway (then may leave by shifting to another plane), however, the elbow (and hence the right forearm) will come back on plane (or at least should) during the impact interval....especially if our hands tell us where to go. Since we properly established our impact alignments at Impact Fix, our goal is to move our hands back to our intended impact, which hopefully and imploredly should have also included the on-plane right forearm and right elbow.
In summary, you are correct...when your right elbow gets back on-plane during the downswing and during the release interval, your right forearm will also be on plane.
Hope that helps a bit!
FL-John