This is even assuming comments are stored by full URL, not domain. For unpopular sites, you would go through page after page without seeing a comment. As a practical solution users would start leaving their comments on the front page where they're more likely to be seen. Which are even easier to guess.
So you made a private discussion board, great. You could also just make an unlisted board here or host an invite-only forum, among countless other ways.
OPs and gab's extension are meant for a decentralized public comment section for any page. But if all comments are stored on the same server it's even more centralized: In early stages it at least accomplishes moving comments out of the website admin's control. But if you start using it regularly, now you've gone from your entire comment history on each site being visible to that site's admin, to your entire comment history on all sites being visible to one admin.
I get that encryption at least makes it a bit more tedious to violate your privacy. However the whole concept of encryption relies on using secret information as the key. Public URLs are not a secret and not hard for the adversary to discover.
That's a bit naive. History shows that as soon as criminal content appears, you will be held responsible for it anyway. You'll get arrested, sued, raided and FBI won't believe you when you say you don't know the key. Especially if the key is something public like a URL, shared by several people.
The problem is not moderation but concentrating moderation authority. Classical comment sections gave this power to the same guy who makes content being commented on. Rep systems gave power to the majority. 8ch IMO has a promising solution by giving the power to whoever created the board. If you don't like moderation you can always move to another board, or make your own. Problem is that you will have much less content. The solution would be allowing overboard which 8ch devs are too incompetent to set up.
With website comments, my solution would be to let anyone run a server, and let the user decide which server they want to push comments to and which server's comments they'd like to see. So for example you would go to xyz.com/abc and by default see 3 comments from your preferred server based.com, and something like "72 more on cucked.com, 32 more on poz.com, 53 more on others" which you can click to show the other comments as well. No upvotes needed, server operator gets to delete content he's not comfortable storing, but if you disagree with it you can just find another server that won't delete it (or host your own).
There's no third option besides physically attacking the guy asking. His main goal is that he gets to beat someone. He is only asking the question for additional amusement. He is not going to deny himself that just because you figured out a clever response. If he was honorable or an intellectual he wouldn't be in prison asking riddles about cock chairs.
I guess you got confused when I said report and thought I meant some kind of rep system. All it takes is someone going to xyz.com/abc, seeing CP in comments, then emailing the db admin with the URL. Now the admin knows the key. Is your plan to just hope nobody will expect the admin to do anything about criminal content hosted on his network?