Ethernet Switching Problems
You can't do this automatically, that I'm aware of, but you can make it simpler:

Setup each ethernet as it's own location in the Network preferences pane, then use the Apple Menu -> Locations menu to chose the location when you switch.

Turns it into changing the network cable, and a single click.

http://osxdaily.com/2013/07/01/change-network-location-quickly-mac-os-x/

OR:

If the internal dev ethernet is only internal (no external access), just have it bypass the proxy for those hosts and domains, and leave the proxy settings in all the time. That way they'll only use the proxy when on the open network going externally, or vice versa.

OR:

Get another ethernet interface, keep both open all the time, and configure each appropriately. USB->Ethernet converters are available cheaply.