I should also make clear that I don't think it's that odd to have a sequence that moves between Main and Discussion as appropriate- but I don't think there are many (or possibly any) examples of that yet, and so it may be odder than I think of it as being.

I can see the point of having the main parts of the sequence in Main and appendices in Discussion, but having Part 1 in Main, Part 2 in Discussion, Part 3 in Discussion and Part 4 in Main (named that way) seems confusing to me.