[ronnie]

Partially answering my own question: We found a way that works. It's a little clunky and we'd still be happy if someone has a better idea or if some update will make the process easier, but for anyone attempting the same, here is the steps we follow now:

• He synchronizes his whole library to a folder, using the function "Sync Library -> Synchronize to a cloud folder"
  
• I export everything I want to share (songs + metadata/audio) to a .msf file using "Share -> Export as .msf", and also export the setlists I want to share
  
• He imports this .msf file (and the setlist file) using "Import -> Local File" with the option 'update existing song' applied to all duplicates, which updates all the songs in his library that are also in the msf-file but doesn't touch the rest. Now he has the correct meta data, pdf files and audio for each song, but lost his annotations if the song was already imported
  
• He uses "Sync Library" again, but now with the Sync Type "Update Device" and only the option "Sync annotations" checked. Audio Files should be on "Ignore Audio Files" and Merge Behaviour on "Always use data from folder". (Note that his old pdf files are transferred as well as annotations, but meta data and audio is kept from the imported msf file)
  

Amazingly, this also worked although his song titles / file names were slightly different than mine. Good job here to check for similar files, Mike!

As I said, if anyone has an idea how to make this process easier to apply more regularly, we're still open for it.

Cheers!