1) After further testing I have discovered the following weird behavior. I have added 2 root items other than the default "Audio" item. I can't access either of the new root items unless I add the string "Audio" to just one of their names??
2) Do you have any idea how I can get my Smartlists to re-evaluate without requiring a restart of MC on the server?
It looks to me like the Onkyo is making those decisions.
Assuming you are using at least the stable version of MC 17 or newer, the updateID is being incremented when smartlists change. This informs the renderer that its cached information for that content directory is obsolete and should be discarded which is appears the Onkyo is not doing.
Also, based on your findings, I'd say that adding items at the root is restricted to the 4 base containers, Audio, Image, Video and Playlist. Those container ID's are fixed at
1,2,3 and 4. I'd assume most devices will not allow adding of containers at the root level.