You might have Cloudflare add a request header to the origin request, like x-cloudflare-key: <somesecret>, and then configure nginx on the server to block everything not containing that header.

For now yes, but over time probably no. Multiple communities around the same subject are created on different instances. They'll compete to become the most active one. Then everyone will subscribe to that one, and it will grow even more. However, if over time you don't like that one any more (e.g. don't agree with the mods) you can start a community with the same name somewhere else and compete again to become the most active one (or not, and stay small if that is preferred).