I completely understand the confusion here, and when you start to look at things more as physical motions, I think a lot of the confusion will be cleared up. For example:
The type of motion you’re referring to can totally be used to play continuous “one note per string” phrases. You just have to do it… continuously! This is what it looks like when you do that:
Of course, when you do this, it is no longer just a type of occasional adjustment, the way we used to describe “two-way pickslanting”, but in fact becomes a picking motion unto itself. A double-escape picking motion. As you can see, it looks pretty different. If we want to ask why “two way pickslanting” players do more scalar stuff and not these kinds of phrases, my guess is that’s the explanation: it’s pretty different, and doing it would look and feel like learning a whole new picking technique.
So yes, “two way pickslanting” is a style of playing where you make single-escape motions, with occasional double-escape motions as connectors between them. The fact that those connectors utilize what appears to be a slightly different physical source containing some forearm, is mostly irrelevant. It’s just a different kind of motion that you’re inserting between the other ones.
The motions themselves are relatively straightforward. They move a certain way and use certain joints. Describing why players choose to combine them into sequences or styles the way they do is really where the confusion started here. “Two way pickslanting” is a style, not a motion. “Crosspicking”, as we used the term, was a style, not a motion. We’re going to drop that way of looking at things and stick with motions just so the actual thing that you have to learn to do is as clear as possible.