As you can probably tell from the long list of unanswered issues, I clearly don't have time for this. This Github project will now be put in read-only mode.
Since 2018 I have been working full-time developing Thunderbird, and even if I did have any spare time outside of work, the last thing I would want to do is maintain more code, especially as it's the same tools and processes as my day job. I stopped using Open With myself several years ago, so there's not even anything in it for me.
Thank you to everybody who has supported Open With over the years, by donating, translating the text, sending pull requests, or helping others.