Your welcome. Yes Orbi uses a single SSID for complete roaming and supports 'bandsteering', Bandsteering aids in helping the device to select the best band. As for the use and your concerns about using DMZ, This does not make things more complicated in most cases. All the port forwarding is done on one router, The router responsible for that subnet. That said a double NAT could cause you problems with VPN but testing would be needed. Yes I do have FiOs quantum TV service, Y do you say you cant hang the VZ router off a 3rd party router in a DMZ? I will admit I did try this a couple of years ago when I was using Sophos and it did not work but I think I had a configuration problem aka user error and that's y it did not work, Others here have reported they have successfully hung the FiOs router off a DMZ and it works fine, Testing would be needed. I plan on testing this myself in the future when time permits.
Orbi looks very interesting and very well could be the most feature rich mesh system on launch, The only thing it seems to be lacking is 'Ethernet backhaul' at this time but many are pushing Netgear to add this on there forums. Netgears claim for this missing feature is Orbi has a 3rd dedicated 5GHz radio like Eero has for the sole purpose of the mesh communication for all nodes so you wont have the 20-30% performance hit that you get without it, My argument is nothing will be as good as wired. I am very excited about Orbi but I am waiting for some reviews from the likes of SNB and others to see if it holds up to its claim. What ever you decide to do, Please consider the security risk of keeping your personal home network on the same subnet of Verizon and try the DMZ configuration's I talk about above, I would test Orbi as the main router by cloning the MAC address from the FiOs router WAN port, This will save you a phone call to Verizon to release and renew your address then hang the VZ router off the Orbi DMZ and see how it works, This configuration will solves any problems you could have with a double NAT and your VPN. If this setup brakes something then do the revers.
What ever you decide to do please post what you ended up doing and the results.
Hope this info helps....