i have an Ethernet connection from the ONT and use my own 3rd party DLINK as the main DHCP, the Fios Westell and my Linksys are just connected via LAN as a bridge
it sounds like you're making it way harder than it needs to be, by configuring all 3 routers to communicate through their WAN ports. which is what i tried to do initially just to save those 1-2 LAN ports
all you have to do is connect the Fios router, just like you do your 2nd router as a bridge through a LAN port of your Netgear N600, which will be the only hardware handling the DHCP and assigning IPs to everything, the other 2 routers should be connected by a LAN port.
all the port forwarding is unnecessary, just reserve the IPs(which ever ones they're assigned automatically) for all your devices and you'll be set to go
the Westell Fios router seems stuborn that you cant actually disable its DHCP and still use it as a bridge, so it will assign its own IPs but it wont get past your router because the IPs arent assigned or recognized by it, the Netgear IPs assigned for your STB's will go through as normal
same goes for anything you plug in to the Fios LAN ports, your Netgear will assign IPs
id recommend putting the STBs as the highest QoS or else you'll get alot of delays with guide menus from packet latency