"the defining characteristic of Promenade Position is not parallel shoulders"
The problem here is that your search for a "defining characteristic" is going to lead to getting the emphasis in promenade all wrong.
The point of promenade is not to make a point of the fact that you are in promenade - instead, the point of promenade is to dance seamlessly with your partner, in a situation in which you both happen to be using your legs in a diagonally forwards direction, rather than the more typical one forwards one backwards. If you make a big deal of it, you are completely missing the point - the defining characteristic of promenade is that it still has to be good, clean, well partnered dancing. If you call attention to the fact that you are in promenade, then you've done it wrong - you've let something trite and incidental overshadow the flow and artistry of your dancing.
Doing your utmost to achieve a parallel topline is the simplest explanation of how you would go about dancing such a good, clean promenade. Nobody will be perfect, but that is the goal to work towards, because that is the goal that makes promenade not something odd, but simply dancing.
Jonathan made a great point when he commented on the situation in which he opened out slightly before the step through, and then fixed it with a rotation towards his partner at the end of the step through. This is a very common fault to one degree or another, caused not just by the need to make room for the legs, but also by our tendancy to bring the trailing side of our body forward when we bring the leg through. The English coaches make a big deal of having you work on swinging that leg across the body, for the man this would be the feeling of the right knee going up and across towards the left shoulder, as opposed to having the right side bring the right leg forward, which would implicitly result in undesired opening out.
But this common issue also highlights a basic element of the technique: most promenade actions specify CBM for both partners on the step through in PP. And that CBM is there not only to initiate turn (for one partner it's in the wrong direction to do that), but to insure that the bodies rotate towards each other rather than away from each other as those inside legs go through. This mutual CBM is only ommited in the figures where one partner is on the inside of a sharp enough turn that the tendency towards opening out that would ordinarily be objectionable is useful for initiating the desired turn.