Is it possible on that +1 be seen only by the owner of the post in Google+

You can see the whole history of the Post via the Ripple of the Post: https://plus.google.com/ripples/details?activityid=CzWHefUKR1T

You can get to the Ripple for popular, shared Posts by clicking the Post's context menu (the down arrow on the right) and selecting View Ripples.

I think in the post you described, Peter Norvig is the author of the first post. He may have cut & paste the "patient zero" posting instead of sharing the original, if "patient zero" was the first?

Only the people that can see the original post can see the comment.

See Who sees the content you post in the stream:

When you +1 or comment on a post, whoever can view the original post will be able to see your +1 and comment as well.