Hmm, yes I see what you are coming from in avoiding the Draft folder that way. Aside from the heading levels matter itself, have you considered making heavier use of the Document Bookmarks feature? I realise that doesn’t entirely address the desire for avoiding duplicate hierarchy, but something to consider is that you could have a structure like this instead:
Then let’s say ‘section b’ wants “Support Material” to be conveniently accessibe. We open the inspector to the Bookmarks tab and drag that into the “section b” document bookmark list. Now it can be viewed in the preview area below, or easily opened into a split view in the main editor, depending.
And now you can organise your supporting material more topically (or by whatever criteria makes sense for them), and even more easily use the same material for multiple sections.
In other words: the design of the software is such that it is meant to “flatten” research and writing content without having to coerce it all into a single outline.
That all aside, I think it will be difficult to get exactly what you want the way you describe having it set up. There is in fact a way of using the current selection with the outline structure, but it involves using the Filter tool (the funnel button to the right of the main group selection). That is how I would most often use Current Selection, which is one of the filtering criterias available. Instead of building a fresh new list from the criteria it subtracts everything from the master list that doesn’t match the filter, leaving the original structure intact.
But to use a filter like that you need a structured selection to begin with, and the only feasible way of getting that is via the Draft folder. So that’s what I mean about it not really helping you out.
I think, to make your approach work, you will have to settle for forcing hash levels by Section Types and Layouts.
As for whether we could modify Current Selection, with the Include Subdocuments checkbox—yeah, probably. It’s worth consideration, but I would caution that in general, compiling non-Draft material is a bit of a niche approach. I would strive, wherever I could, to find a way to make use of the Draft folder given its advantages—perhaps using something like the above as a way of working with supporting material.