So, does this mean a remote user cannot access nor even see what’s stored, locally?
In the context of this thread (DLNA media server):
A remote DLNA client cannot access the DLNA media server (since, as far as I am aware, DLNA does not support the concept of a remote client).
There are applications that create proxy servers for particular media players. For instance, BubbleUPnP has a BubbleUPnP Server which acts as a local DLNA client, but also acts as a server for a remote BubbleUPnP client, and allows remote media streaming from the DLNA media server.
Nazar78 has very kindly built BubbleUPnP Server than can be installed on the MyCloud:
In the wider context, you can use the WD apps to gain access remotely. These don’t use DLNA, though.
And, do local users see everything & everyone’s streamable content?
DLNA does not respect access controls (that’s a DLNA specification issue, not a Twonky or WD issue), so, yes, if you enable media serving on a share, any local DLNA client will be able to access it.