Re: Google WiFi Mesh & Verizon Fios Issue

No. Turning off wifi is just disabling the radio. You should always disable the Verizon router wifi if you are using a mesh network. If both routers are using the same channel that close to each other they will be interference. 
The white wifi light should be off when it’s disabled. 
Bridge mode would be disabling the DHCP server in the Verizon router. Then changing it’s IP address from 192.168.1.1 to something in the Google’s subnet range. 
A double NAT is not an ideal situation but won’t necessarily cause your wifi to drop. When you notice your wifi drop are you able to hardwire a laptop to one of the LAN ports? This way you can tell whether your internet connection is dropping of it’s a wifi issue.