Its best to try think of the various WAN connections not as A/B/C/D but as what is in various priorities in each unit, and if possible I’d strive to connect both the Comcast and CL connections to both 20X, how you do that may depend on the handoff you have from them though, but it could be as minimal as a couple of dumb switches and adding a USB-Ethernet dongle to each 20X.
Set that to Priority 1 then in your config, put the Cell into Priority 2.
Config replication will do the same to the priority of the connections on 20X-2 so all good there, you may want to tick the box on the HA that keeps the standby connections alive.
Correct, just put it into a lower priority than the Ethernet WAN.
Yes, failure of 20X-1 should result in 20X-2 taking over, in testing depending on how the failure happens that’s normally a few seconds for me if I have a switch connected downstream to both the 20X LAN side and connect a test client to there and start pinging out.
They inherrit the same priority settings, so what you configure on 20X-1 will apply here unless you disable config sync and do things manually 