According to /u/Spez, Reddit CEO, the reports of messages edited without any user consent or knowledge are correct as he admits to have done it so himself:
Hey Everyone,
Yep. I messed with the "fuck u/spez" comments, replacing "spez" with r/the_donald mods for about an hour. It's been a long week here trying to unwind the r/pizzagate stuff. As much as we try to maintain a good relationship with you all, it does get old getting called a pedophile constantly. As the CEO, I shouldn't play such games, and it's all fixed now. Our community team is pretty pissed at me, so I most assuredly won't do this again.
Fuck u/spez.
The edits were made in a thread linked from the Washington Post which described the recent ban of the /r/pizzagate subreddit which tried to uncover child-molesters and recently moved to voat.co.
(Score: 0) by Anonymous Coward on Sunday November 27 2016, @01:29AM
Comments should really be signed blockchain-style (as long as your comments are numerically increasing it should be fine to just use that as an index.) with the blockchain signatures for all comments distributed ala bitcoin. While this may sound silly, given sufficient encryption (or better yet multiple ciphers signing the messages to ensure all keys would need to be broken, not just one.)
So long as a copy of the original message db survives, along with a copy of the blockchain, all messages could be authenticated as originally published, regardless of the anonymity(or lack thereof) of the poster, and without needing to trust the future maintainers of the site, so long as the already published blockchain remained online and outside the realm of a majority signing node compromise (same issue as bitcoin.)