Mr Rhodes, you last suggestion is correct, you can essentially just leave the firewall pair in place and simply have the "master" connected to the WAN. If the master fails, you would lose internet, but your local LAN would still function.
I still do not believe the Y cable with work. The problem I see is I am unsure if the router will be able to establish Layer 2 connection (namely valid ARP assignments). when you run through a switch even a dumb switch, every port has a single MAC address connected to it. The router then knows, hey to reach MAC address "2" and "3", I need to send it to MAC address "1". On the device with MAC address "1" its ARP table has "2" and "3" on separate ports that are directly directed allowing proper packet routing.
Your Y Cable is attempting to connect multiple MAC addresses to a single port and I am unsure how the router wand firewalls will deal with that.
