To make sure I am understanding and that we are talking about the same thing, when you say "sharing" are you talking about the "share this email" portion that is included by default in many of our templates? Specifically the email icon for sharing with a friend?
If so, I can confirm that you cannot share a campaign to an opted-out contact using that feature. We won't give an error message if you were to try, but the message won't be delivered to the opt-out contact.
As far as why shares aren't appearing in your "shares" tab, I'd like to get some more info. Can you walk me through the process you used to share this mailing? Once I get that info I can do some digging on why it may not have been tracked.