I'm not using Zoneswitch at the moment, and I haven't used it with a DLNA Renderer, so I can't say with absolute surety that I know what I am talking about, but...
Playing Now playlists, which are created when you add something to Playing Now by one of the various methods, are associated with just one Zone. So if you have a Playlist of mixed DSF and FLAC files, when you add that Playlist to Playing Now, it is going to add the whole Playlist to the one Zone. Probably based on the first file in the Playlist. You should be able to test this and see what MC is doing.
Continuing that idea, if you have a Playlist of say FLAC files playing to the Marantz, then try to add some DSF files to Playing Now, you would not want MC Zoneswitch to switch to the Oppo immediately. Hence the new tracks would be added to the current Playing Now list associated with the current Zone. I'm not sure if Zoneswitch would then get activated when Playing Now reached the new tracks, but I think not.
If playback of the FLAC files was finished, and you added DSF files to Playing Now, I would expect Zoneswitch to select the correct Zone. If that isn't working then the "Stop playback in these zones" setting may be the issue.
I have seen reference to Zoneswitch not working with mixed track formats, and specifically with DLNA Renderers. But finding a reference to that in the forum may be a challenge.