Who I have the same questioned this solution

That should work.  A few points:

You said "modem."  Hopefully you're not using it anymore.  If so, remove that puppy from your system.  It has no use or place in a fios system.

Verizon routers create a MoCA LAN on the coax.  They have a built-in MoCA adapter.  You only need the one adapter at the Orbi.

Check if the Orbi can be run in Access Point mode.  If so, you won't create a 2nd network.  Devices will be able to see each other regardless of if they connect to the Orbi or the Verizon router.  This can be useful if you have shared devices like printers, NAS, or media servers.

Make sure the "whole home" splitter is passive; i.e. no amplifier.  Amplifiers can interfere with the bi-directional MoCA signals.  Some are specifically designed to support MoCA, if so, they should be labeled.  If you have problems and have an amplified splitter, replace it with a passive splitter.