If you want to use the PI as a router you'll probably end up with a double NAT situation which isn't ideal but may work well enough. In terms of wifi performance, I wouldn't expect a Pi to be particularly good here so I'm not sure this even worth it unless it's just a budget issue and you don't have any other options.
In terms of your problem, you should be able to assign the Pi ethernet port to the default WAN and WAN6 networks. As for wifi, the Pi adapter needs to have support for AP mode, and looking around it doesn't seem clear if the built in wifi adapter supports that or not (most people using the Pi are using it purely as a router and not a wireless AP). If not, you'd need a USB wifi adapter that supports AP mode. You might want to get that additional ethernet adapter too for testing/debugging and it will allow you to add a dedicated wireless AP.