Glad to hear things are moving in the right direction! The issue you’re describing with the URI handler is a known issue that’s been affecting some Windows users. For now, if you haven’t already, try closing and reopening your browser. I’ll do a bit more digging tonight when I have the time.
Do you happen to remember if you ran the extension in Aseprite before or after tying the “Open in App…” link? I ask because the URI handler relies on a value that gets set by the extension when you run it in Aseprite, so you’ll need to do that at least once first.