(You mention they exist, but here's an open source plugin that does comment folding and nothing else: https://github.com/andrewheins/HN-Comment-Hider)
I've made a few tiny edits so that it works with https (the github repo has this, but not on userscripts.org), as well as the threads page. Also, I put slightly less space between the [-] and the commenter name, it looks a bit better/more reddity.
I didn't bother making it work for anything but the greasemonkey script, or trying to upload it to userscripts.org. But it might still be useful: https://github.com/zwegner/HN-Comment-Hider