I agree pretty much with your post, and I think these are important points for those who truly want to grasp the unique characteristics of Yang Style and Chen Style. The most recent opportunity for me to consider this issue intensively was at the Tai Chi Symposium in 2009.
At the Symposium, what I understood from the Chen representatives is that they believed that Yang Style and all Tai Chi styles have silk reeling energy 缠丝劲 (chan3 si1 jin4). As I understand it, this "reeling" or spiraling motion originates at the Dantian. However, I do not recall any representatives of the other styles using this principle in their teaching. Instead, the only references were to such things as "mobilizing the Jin like drawing/pulling silk" 运劲如抽丝 (yun4 jin4 ru2 chou1 si1), "leading movement with the lumbar region/waist," "moving in circles to keep the movement continuous," "and sinking Qi to the Dantian."
I also felt that the logic of the Chen principles seemed to require subtle differences in the timing of arm rotations, so that a Chen ward off tended to differ in execution from a Yang ward off.
My own current view is that "reeling silk" and "drawing silk" refer to different principles, as you have articulated in your post, and that Chen and Yang style look at movement through somewhat different prisms. I think that understanding some of the Chen principles can help with Yang Style, but that adopting too much of them merely changes the flavor of the movement from Yang Style into Chen Style.
What I understand from a Yang Style perspective is that externally, movement should originate from the waist and internally, from the Dantian. The movements should generally proceed in circles; therefore the waist must generally move in a circular fashion, sometimes horizontally and sometimes vertically. In a few cases in the form, the only way for the movement to feel continuous is to move the waist in a subtle spiral or figure eight, but this is not something that is a necessary focus at most times. In Push Hands, you may need to focus on an explicit circling of the Dantian if your opponent tries to put energy there, but again this is not the norm. You need to sink Qi to the Dantian so that it can support the internal movement of the Jin, but you should not force this sinking or focus on manipulating the specific movement of the Qi in any particular pattern. You rely on the fact that the Qi will follow wherever you put your Yi 意 (mind intent) and according to how you put meaning into your movements.
This is all I have time for now.