Sub folders can only be used to group receipts by account, for example if you are uploading to multiple QF accounts from one Dropbox (as explained here).
That is correct, however you can save the purchase and edit after creation to itemise across different nominal accounts.
Regarding point 3, Dropbox is good for bringing in larger quantities of historic receipts, but I use it with CamScanner to bring in new receipts too, works perfectly well.
I’m not 100% sure what the purpose of sub-folders is in Dropbox as it’s mainly used as a conduit for importing receipts into QuickFile rather than a long term archive.