The only reason I can see for going down the Flow 2 and Flow 2a route is that it avoids a need for cautionaries - I can’t see any other reason for doing it. The downside is that a flow is no longer always an independent piece, it can also be one piece made into two, or more sections, purely to appear correct.
I remember a brief conversation I had with Daniel before Dorico was launched. The subject was merging Word Processing and Music Notation. He told me that it was an objective (I’m fairly sure he may also have said long-term) for Dorico. To be honest, I think they’ve made more than a fair crack of achieving that from the start. There’s a huge amount you can do already - and it looks great and is easy to work with.
Merging Word Processing with Music Notation isn’t a niche thing. Away from all the scores and parts, there are thousands of textbooks out there that mix the two together. Most of them are hundreds of pages long. Some of them are centuries old. And as far as I know, all of them organise themselves into chapters - that’s their way of defining “independence” within their work.
On the surface, separating content into flows may not seem unduly onerous. But if you have a chapter separated into twenty flows, and you have twenty chapters in your book - the amount of flows in your work is, being blunt, ridiculous.
As I said before, I don’t see this as in any way urgent. As david-p has said, one can already do this by using two separate programs (which I have, often). But it would be better, and so much easier, to do it in one.