Okay, I’ve published the test results here:
https://codeberg.org/scy/python-markdown-roundtrip-test

Summary: If you need to do #Markdown round-trip modifications in #Python (i.e. load a Markdown file into a Python representation, make some changes, save it again, while _only_ changing the parts of the document that you actually touched, not normalize/rewrite it from the ground up), then #mistletoe is what you want to use. #Mistune is a close runner-up, but has its issues. Everything else basically doesn’t work.