For me, opening and closing are slippery, but ultimately simple concepts. "Opening" means things coming apart or expanding, and "closing" means things coming together or contracting. In PWHM, I cannot say that I feel my shoulder blades opening up internally or externally in the final position, but rather the opposite. I am still unsure why i would even want to feel them opening, since it would seem to weaken my energy. What I do strongly feel, however, is that the opening of my hands comes from the action of the strong muscles in my back. Perhaps, this is the feeling you all are describing.
Using strong muscles, rather than weak ones, to generate power has been a major part of my practice and teaching of late. It is how I interpret many of the classic injunctions in more concrete terms.
As for the difference between FD and PWHM, I find the former bigger and snappier. The latter feels more deliberate. As I understand it in our form, the former trains striking energy more, whereas the latter gives more scope to Ward Off, or possibly Shoulder Stroke, Press, or Elbow. In the former, I will give up all the energy on contact from a long motion, but with short energy. In the latter, I will use shorter arm motion to deliver longer energy to uproot and then launch or throw my opponent.
As for spine versus back, I think that we would describe all these issues as part of the external aspect of generation of energy. The internal generation is more connected with the Dantian, the lungs, the meridian networks, and the connection between spirit, intent and Qi. The external, however, has again both external and internal aspects. I would call the shape of the chest and back external, but the energy produced in that shape, internal. An arch, a whip, and a bow can all assume a similar external curve, but the energy inside is different. I think the masters talked about drawing out the back to address shape and about sticking Qi to the spine to address the energy produced within that shape.
When I issue, the energy will visible travel from bottom to top. The last place it gathers before the final snap is in the soft tissue in my upper back that connects my spine to my shoulder and arms. If my back is not rounded or my shoulder is bent backward, the energy cannot gather there. It feels like trying to feel energy in an unstrung bow. If, on the other hand, the relationship between my chest and upper back does not change, it again feels like the energy cannot be concentrated there. It simply moves through, like pushing on a spring that is already compressed. This is also the point where the vertical flow of energy transforms into a horizontal flow. The external aspect of the energy bounces off the root, goes from back leg to front leg, branches off to go up the spine, and then splits off into one or both arms. If you do not round the crotch, the connection between the legs is broken. If you do not loosen up the lumbar spine and drop the butt (松要落跨), the connection between upper and lower is broken. If you do not draw out the upper back and enfold the chest (含胸拔背), the connection between left and right is broken. Where you last feel Qi concentrate in the body is where left and right meet, in the upper spine.