Or have an auditable edit history.

I like that suggestion even more than mine. Could be a link beside the timestamp, like "4 hours ago (edited)" and clicking the edited stamp would give you a history. I edit my comments all the time and wouldn't want to lose the ability to improve my wording or fix typos or whatever, but I don't see any harm in that transparency. The ability to completely delete a comment (along with any history) should probably be preserved though.

That might work if you could link individual comments to a version of their parent, otherwise confusing over comments not making sense in the current context would still occur, since most people aren't going to read an edit history first.

The very least they could do is add some visual cue that a comment has been edited, like showing the header in italics or adding an asterisk.

I think a visual indication that a comment was edited, then diffs of all their edits, would be very nice to have. I have run into (and caught) people that make provocative comments and then edit them to make the people responding to them look foolish.