I wait to be corrected, but I think that when you import a Flow, it’s mostly just the ‘music data’. So an object’s Properties, like Scale (as overrides) should come through, but Layout info, Master Pages, etc pertain to the document.
I would certainly recommend putting the book together as one document, with songs as each flows. You can always export/import Master Page sets from one document to another.
TBH, you should be able to do it without many manual overrides. Check that there aren’t global options for setting the position of Rehearsal Marks, Tempo marks and System text, etc, so you don’t have to move them about. The same for Note Spacing and Staff Spacing.
Actually, I shouldn’t have included master pages in the list - these songs all use the same set.
I did a quick test, and indeed, system and frame breaks were lost, as were changes to chord symbol appearances. I didn’t check everything in my list - these alone were deal-breakers.
At this point, I mainly have to adjust some font sizes. It will probably be faster to do that in separate files than to combine them and re-do the overrides. There are a good number of system and frame break overrides as well as some in note spacing and staff spacing.
Next time, I’ll import the flows at the start. That would have saved time overall. Lesson learned!