Apart from discovering that Webmin stopped Squid support in Squid 2.5 and now needs to be updated for Squid 2.6 I was never able to get Squid running in reverse only mode. (These are the Reverse proxy settings. Squid also seems to have changed it's manner of working on what ports it wants to use for RP)
Even though the .conf is probably one of the best documented .conf files out there it was so long that it was almost impossible to work out what went where!
What would be incredibly handy would be for Virtualmin supporting Squid (even if it's to just ensure that setting are correct for reverse proxy use. (Possibly it would handy for Virtualmin to ensure that hosts / apache.conf etc are configured for reverse proxy use). Getting Squid to work as a straight proxy server was no issue for me and very straight forward, but getting it to work in Reverse only mode defeated me totally).