"I'm not being critical of signal/response leads, I'm just pointing out that this is one of them."

I would disagree - it's not purely a signal.

"What separates the signals we designate as the skills of learned dancers from the signals one might think of as a trick or a cop-out, such as the silly examples I proposed above?"

If the component we consider the lead is a joint participation in the physical creation of the action, then it's more than a signal. There may need to be additional actions done by the lady alone - her knowledge is more of the followthrough than possession of a secret decoder ring.

So for this develope, you don't describe it as a body action, and it's not one of the tilt the torso back to raise the leg variety, but there's still some small but critical action in the core as it begins. And that action can be done together.

Maybe you will still call it a signal - but it's a signal that's physically involved in the early stages of the action, rather than one that is arbitrary.

We might also question the wisdom of using figures that appear to come from nowhere not just in their visual impression, but in their explanation. A lot of dancing is illusion, but if the people trying to do the dancing are among the illused... it's not going to be easy to do. One of the problems with a lot of social dance material or the way it is often done is that it's not all that evident how basic principles can be applied to create it, so it ends up being learned as something arbitrary rather than something with its own flow of each action implying the next.