it would be interesting to know if the browser is able to pick up the proxy.pac. So I would pick a test machine, install Wireshark on it, start a packet capture, launch the browser and access google.com. You should see how the browser talks to MWG and loads the proxy.pac. You should also see the response. If you see the proxy.pac being sent to the browser, maybe there is a syntax error somewhere and the browser will not use it.
If the browser cannot load the proxy.pac from MWG, we would need to find out why.
Maybe you can create a capture and share it?
A more straightforward way to check if proxy.pac is configured properly (using the parameters in the original post) is:
1. open a shell (on unix) or a command window (on windows)
2. execute these commands:
telnet xxx.xxx.xxx.xxx 4713
3. Hit ENTER twice after the GET command. If you see your proxy.pac contents fly by, then you're pointing to the right place, and xxx.xxx.xxx.xxx is properly serving up the proxy.pac.
FileInsight is a nice tool that can look at the raw data from a URL.
It's used for many other things, like webpage malware research, but I use it for the purpose you describe, as well.