ISA 2006 Single NIC Reverse Proxy configuration questions

I have a system (VMWare ESX with single virtual NIC available) with Server 2003 and ISA 2006. This client is moving this week and need to setup an inbound proxy between the internet and inside. I can set firewall rules for OWA/OMA, and two inbound ports (one for 8080 and the other 7777). Once the traffic is in the DMZ (on a Snapgear) , I need to setup the ISA to then send it to the internal IP (I can route through the firewall potentially). This needs to be done for a few ports for OMA/OWA, 8080, 7777.



An ASCII diagram:

                               INTERNET ----- Firewall ------ Internal (192.168.5.0)
                                                          |
                                                          |
                                                          |
                                                      DMZ (192.168.4.0)

Questions:

1) Can I use the HTTP on a different port (like 8080) to pick up that traffic and forward it? I get a refused screen when I try to set it up that way.
2) If I forward the port 8080 to port 80 to the ISA, I get a Forbidden 403 with error code 12202. My rule looks fine as it is listening on the internal network, I have the hosts file setup to forward acme.com to the DMZ port.
3) Two NIC option is NOT a solution (long corporate story about needing to do this with one nic), so if this doesn't work, can you think of any way to have traffic come to the DMZ and then rerouted internally (even with just Windows routing table or something along those lines)?

Thanks.