I agree that the shoulder pack is the healthiest position for the shoulder, and should be an active part of all movements. That being said, when the arms are overhead, the scapula must protact. Protraction is not some evil movement that was cunningly placed to make our bodies hurt, it is an vital part of how the scapula needs to move.
The shoulder pack is optimal shoulder position, not holding the shoulder back and down.
I don't think the push up plus is necessarily bad, but there are much better ways to train scapular movement to generate stability (arm bars, get ups, windmills, pressing ,waiters walks). These seem to be more intuitive to the function on the scapula and rotator, correct positioning and holding the bits in place.