1.Yes, CentOS 6.9 is supported.
2. Not recommended and not supported by Kloxo-MR. Nginx does caching pretty well, why put Varnish put in front of it? You are introducing overhead with 3 layers, keep that in mind too!
3. Yes, TLS session caching is supported by Nginx.
4. On my CentOS 6.9 and CentOS 7 Kloxo-MR i got pure-ftpd, nothing else.