1. it's described in the link i posted
2. i suppose this to not affect the mechanism, as it address is carried inside the http request
stunnel is not an http proxy, as apache can be, and so you have no way to get the real ip.
with nginx (i'm not using apache) you only need a line like this:
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
as an alternative, HFS 3 is already supporting https natively, one may not need a proxy.