This all seems like normal behavior... You don't need to enable DLNA on Clients in order to connect to the Server using access key method. You can turn off DLNA on Clients, unless you really need it ON for something else (like DMS TV Tuners).
I agree that DLNA is not needed on the Client to connect to the Server. Without DLNA I can connect using either the Server's Access Key or IPAddress:52199. Note the URL must include the server's Library Server Port (usually 52199) when the library is first added to the client under Playing Now. This piece is not obvious from MC's Add Library dialog box (see attachment) or its error message.
Don't you need to an "Update Library (from tags)" to populate those fields from the Vorbis tags? Just creating a field won't populate it.
That option is grayed out on my client when connected to Server via DLNA. Also, the Vorbis tags do not include the Server's Expression Fields, which the Server cannot save to tags. If I re-add the Expression Fields on the DLNA Client, they don't get populated. As previously stated, I find the fields available on the DLNA client are a very limited subset of those on the Server (unlike non-DLNA connection).
There are many things you just can't do from a client, like changing/saving cover art and creating tag fields. Of course, if the tag field already exists on the Server it can be changed/saved from a Client... But I've only used access key with authentication method connecting from Clients to the Server.
I also find the tagging operations to be very limited on the DLNA client, and are completely absent for Expression Fields. Guessing this may be by design, since non-MC DLNA clients may not be able to handle arbitrary fields defined by MC? I also find the Library Views available on my PC DLNA Clients are very limited, generic, and do not always sort properly, even though I configured Custom Views in the Server's DLNA setup. If I try to load a Custom View from file on the DLNA client it does not change anything. Is this intentional because non-MC DLNA renderers cannot handle arbitrary MC Library Views? The identical Custom Views setup for JRemote,Gizmo,and Panel function nicely and sort properly from Android player apps.
Personally none of this holds me back since the only "DLNA" renderers I have are actually old Chromecast audio dongles spliced into MC through a BubbleUPnP Server (presumably looks like DLNA to MC), which works just fine. But it helps to understand the differences.