As a personal trainer plus Anusara Yoga instructor, scapular retraction is key and one of the elements I bring into my training DAILY. I applaud you for bringing this area to the forefront of training.
The video of the woman going upside down against the wall. In yoga, we call this L-pose and I use it all the time in training. A few tips. I place my hand on their sacrum and lightly apply pressure (towards the wall) to help them feel secure. I might further retract their scaps with my hands, then have them hold this on their own so they can really feel this action. Standing with arms overhead in the middle of the room or in L-pose.
The full plank or half plank retraction, I call shoulder flossing and also is a great rehab for shoulder injuries provided the hips are stable and don’t move.