Question

Each comment is a separate mediawiki page, with its own history. You can use revisiondelete on that, but usually you'll just delete the comment (revisiondelete should still be used to hide the deleted comment from sysops). Rollback and revert also only works on individual comments, e.g. if I were to edit your comment above, you could undo that. But you can't undo my addition of this comment, you'd have to delete the comment, or move it.