I have been through these problems with Fastmail and found a good solution.
In Fastmail, every folder is actually a subfolder of Inbox. Just look at the folder tree in Pocket Outlook. In IMAP parlance, this means that the "root folder" is Inbox. You cannot specify a root folder in Pocket Oulook, but you can in Flexmail (www.webis.net).
Also, in Flexmail, you can specify just how folders on the 6700 sync with the folders on Fastmail. By trial and error I have found the good settings.
Do yourself a favor and download a trial version of Flexmail and install it. Then send me a PM.
By the way, many if not most IMAP servers are NOT implemented using the root folder concept.

Ed