Security

You shouldn't need to redirect your default web page. It sounds as if your DNS is resolving differently depending on which side of the proxy server you are on. I'm not too familiar with MSProxy but best practice would have your clients never touching the proxy server (assuming your web server is a separate PC). Externally, your web page should resolve to your proxy while internally it should go directly to the web server. If you have a forwarding DNS set up there must be a way to do a clientside host file on the proxy that is never zone transferred outside. any MSP people out there know exactly how? Or if you have a separate DNS set it up there.

Since your web servers are on the internal network the proxy should not even try to handle the request. Make sure that the LAT is configured properly on the proxy. Another to try is put the proxy client on your workstations.

On your client machines, make sure the Web browser is set to "Bypass Proxy for local addresses" (IE).