The page can only be displayed in a frame on the same origin as the page itself. The spec leaves it up to browser vendors to decide whether this option applies to the top level, the parent, or the whole chain, although it is argued that the option is not very useful unless all ancestors are also in the same origin.
Not sure. You should compare with my blog, which is working fine on the latest version of Discourse, and showing comments below the blog post: To Serve Man, with Software
I am pretty sure that header is coming from your blogging software, not Discourse. So my guess is that you upgraded it recently and someone added the extra header.
I was having this same problem (right now, with up-to-the-moment Ghost and Discourse), and fixed it by changing the Discourse site, not the parent Ghost site.
And it was a struggle to get rid of the x-frame-options headers coming from Discourse! I ended up with this in my Nginx site configuration: