Most consumer modem-routers speak DOCSIS, not MoCA, so you don’t want to get one. While you could probably find a ISP modem-router on eBay with MoCA, it would be more straightforward to buy standalone MoCA adapters and a WiFi Access Point.

You would connect one MoCA adapter between a LAN port on the Xfinity modem-router and the coax splitter. Then the other MoCA adapter between the coax splitter and WiFi Access Point.

You may want to replace the splitter with one that can pass frequencies up to 1675 MHz. A standard 1000 MHz splitter will work though speeds may be reduced.

Finally, it’s recommended to put a MoCA filter onto the input of the splitter to keep the MoCA signal confined to your house. If your Xfinity modem-router uses DOCSIS 3.1, a 2nd MoCA filter on the modem can ensure that the modem avoids using MoCA frequencies. Not all ISPs provision DOCSIS 3.1 to use MoCA frequencies, so the 2nd filter may not be needed.

I would like to repurpose a Netgear AC1200 Modem-Router as a range extender/MOCA/Switch to add ethernet ports to our home offices.

Easy enough to test: Can you configure the Netgear modem/router in such a way that allows it to be hard-wired via Ethernet to your Xfinity gateway, LAN to LAN, such that any devices connected to the Netgear, wired or wireless, receive IP addressing from the Xfinity gateway? (typical manual configuration involves setting a manual IP for the AP, plus disabling DHCP services)

If the Netgear modem/router can’t be configured to work in this way, it won’t work if linked via MoCA, either. (Best case, you’ll need a separate MoCA adapter to proxy the Netgear connection with the Xfinity gateway.)