How do I recover from a botched adopted storage migration? I can't take pictures or screenshots
I believe what solved my problem was that I ran Migrate data in the newest adopted storage. Until then the system was in a state where the abandoned adopted storage was the "primary storage", but wasn't present.

I'm not sure why Migrate data on Internal storage wasn't available before I abandoned the first adopted storage. Maybe all I would have had to do was to reboot and the command would have been available.

I am going to move more of the story from the answer to the question, to create a more coherent Q&amp;A picture.