[Yoda]

...How do I use the right forearm and elbow to power the backswing?

...I'm a little confused, and this is resulting in a backswing that is way under turned shoulder plane - my left arm is under or below my shoulders.

Here's a simple little drill that will help your Right Forearm Takeaway immensely:

1. Without a club, assume your address position, and grasp your left wrist with your right right thumb and forefinger.

2. Stretch the Left Arm a bit.

3. Now use your right arm to take your left to the Top. Keep all motion parallel to the Plane Line (the Target Line in this case), and Feel your Right Hip and Shoulder clear to the rear to accomodate that movement.

That's it.

Do the drill several times a day until you get a sense of how the Arms operate to get the Club 'up' on the Backstroke independent of the Body Turn.