RE: question 1, since you are using Varnish 4.1+ you might consider leveraging the PROXY protocol that is present in Varnish 4.1+ as well as HAProxy. One of the advantages of this approach is that it requires less fiddly header consultation to determine the original client IP from the TLS connection from Varnish/Apache (e.g. see this guide). It is also supported by other server daemons & TLS terminators.
When you say “domains are related” do you mean the domain and a sub-domain? They are different domains for completely different websites but they both belong to me and they are on the same Apache server. Both domains are pointed to the same public IP of my vps and Apache redirects the traffic accordingly, so I’m not sure if my domains are related…