[andreaskagedal]

If you are editing a group, the "Remove" option removes the songs from that group. If you tap the three dots at the top right, there is a "Delete Songs from Library" option which permanently deletes them. I set it up this way as users were deleting songs from their groups on accident and then getting upset about it (even though there was a confirmation dialog). 

I understand. I makes sense. Do  not change this.

There isn't an undo option at the moment as supporting that is very complicated, as it would involve rolling back changes in the database (meaning I have to store snapshots of the database, or lists of instructions to reverse every change), and unless I set up a "recycle bin" of sorts with song files, I wouldn't be able to remove those either, but then I'd have to manage when to empty that "recycle bin". I just really don't want to mess with all that unless there is a huge demand for it. You can revert your database to an earlier version as mentioned by Geoff though.

Ok. No problem.

The "Find missing files" (proposed by  itsme) worked fine. for anyone else reading this, it can be found in Settings - Other Settings.

I did not manage to make the "File Paths" option in the search menu work. Even if I long-click a working song, select the "files" tab and copy the file path, and then go back and paste it into the search field will it find the corresponding song. The manual does not give any hints on how to write the path such that songs will be found.

But my immediate problem is solved.