A happy update: both problems solved!
- Indeed, invalid YAML in the front matter to the notebooks was the problem on my project. Thanks in particular for pointing me to the Customizing blog posts with front matter section of the fastpages README.
- I was naively assuming that I could run GitHub Actions locally on my machine, but actually this requires some extra work. Eventually I found your Development Guide and I can now preview my project using Docker containers on my laptop before pushing.
Many thanks for the time you have taken to look at all my questions. I was really bogged down and now I can move on using fastpages. Phenomenal!