Do 10k (and moderator) users have the ability to read deleted comments (are comments deleted forever, or never really deleted like questions and answers)?
No, 10k+ users cannot read deleted comments, so a deleted comment is as good as gone.
The comments do, however, still exist in the database and ♦ moderators can read them and see who deleted them should an issue arise.
No, we (10k+ users) don't have that ability. They are as good as deleted forever (at least to non-moderators—those diamond folk can still see them).