Seems like I stumbled upon another piece to the puzzle... I'm just setting up a win10 (64bit) desktop, and after copying my portable MusicBee 3 install w 3.1 patch over to the new machine, turns out the 'feature', which lead me to believe the above 'workflow'/feature is intended, does not seem to exist / work on Windows10 (at least not the 64 bit version...).
On windows7 (32bit) and the same MusicBee version: If there was a folder in the Playlist Manager holding multiple playlists, selecting the 'parent' folder would populate the file-list and act as an aggregator (= showing the contents of ALL child-playlists it holds - but only 1 of each track, even if there were multiple occurrences of the same file/track across multiple child playlists - replacing this [or one of the occurences in ANY of the playlists replaced them across multiple playlists... which I thought was awesome...]).
Now on Windows 10 & the same MusicBee version/setup, if I now select any of the 'parent' folders, I do not see the 'aggregator' view of the folder anymore (= tracks list remains empty)...
I'll test some more how this behaves when replacing playlists items, and if it still affects multiple occurrences of the same across playlists.
Would be great if anyone, ideally Steven, can confirm which of the 2 different behaviours I have now seen is the one to be expected ?