interesting we have a very similar internal flow - we like working in markdown but our customers want to leave feedback in Google docs, so we also have an md -> gdoc tool. We don't do the reverse as we ask them to only leave comments/suggested changes and we apply those directly to the markdown and re-export.

I ran into similar issues as you for the image handling, and the work around I use is to use pandoc to convert to docx as a first step and then import that as a Google Doc using the API, as Google Docs seems to handle docx much better than markdown from what I've seen.