A few things I can comment on:
- Playing music on the LB website: we have our own music player that integrates with youtube, spotify, apple music and soundcloud. Playlist items don’t have a link to the playable track, we use these services’ search APIs to find a matching track to play. Not anything that can be easily replicated somewhere else I’m afraid.
- However! We have separate tools to find a spotify/apple/soundcloud ID using a recording MBID (playlist tracks always have those; the feed event will contain a link to the recording): https://labs.api.listenbrainz.org/. If you are technologically inclined you could use that to try to automatically resolve each track to a music service of your choice (could work with strawberry+spotify for example)
- Indeed for sharing with friends, the obvious solution is to share the link to the playlist on the LB website.Even non-users can play music from that page (using Youtube), with the caveat that the Youtube search is not always good.
- What do you think is missing in terms of opengraph/tags on playlist page to make them better or easier to share?
I hope I didn’t completely miss the point 
I had not thought of the playlist feeds as a way to play the actual music, but this is an interesting topic.