There is a big issue with that which is design of DLNA/UPnP that is server based and Client only gets temporary access to file stream, no actual direct access to manage them. Shortly said you have one DLNA server and all others are clients. In your Case you have two DLNA Servers (MM and Medion) that are in conflicts where only one can act as Server. To avoid that we started free MMS (MediaMonkey Server)
that should evolve in solution for MM and allow actual DLNA parts to be handled by NAS or any dedicated device that will serve as one centralized server for library and files.
That said we came to second issue which is that DLNA is one way service eg. Server to client where client can't edit, manage playlists and Media on Server in any way it can only play media of server in way that server is serving it to client. Bad thing is that there is no way to tell how each DLNA server implementation will dynamically present its media to clients. Playlist served to clients are generated on request and thus filenames and paths can't be static so next time you reffer to same path you can get different track or no track at all (common that is the case).
HTTP:// is just transport layer and temp link to media stream not actual path which on Windows uses "\" as folder separator and "/"on linux and Windows uses Drive letters where Linux uses Mount points, ... which means that folder on windows can be "t:\Media\Queen\" or "\\MEDION\Media\Queen\" or ""\\192.168.1.110\Media\Queen\" and on Medion folder is read as "/share/md0/Media/queen/" or "smb://localhost/Media/Queen/" or "/mnt/Media/Queen/" and few other ways depending on Medion DLNA Server access permissions.
Based on your setup you have three options:
1. Use MM as sole DLNA server and Medion as storage where you manage all on MMW/5 (MMS in future) and MMA, Marantz just access library
2. Use Medion as Sync Device where you will actually just sync files and playlist and then let Medion Media Server be the one that will serve media over DLNA to MMA and Marantz
3. Find the right settings in M3U export so that Medion is reading them and can serve them as playlists to clients, but that means you will have two DLNA servers and each one on its own way will serve media to connecting clients. It is tedious process to find right combination of settings and you need to know that if Medion or Server app developers decide to change something you gotta start from beginning