Thanks for the information.
I don't run Meraki Go switches, so this is a wild guess. What you definitely need, is the ability to create different VLANs. If you can't do that, it will not work.
If you can:
VLAN1 will be used to allow communication on your LAN, and VLAN2 is for the WAN-failover. VLAN1 runs with DHCP-server, VLAN2 has DHCP disabled.
The LAN-port on the Firewalla is connected to port 1 of switch 1, this port is set to act in Trunk-Mode, Native VLAN is VLAN1 and ALLOWED VLANS are all but VLAN2. The failover-WAN-port of the Firewalla is connected to port x on switch 1. Port x has to be set to Access-Mode with VLAN2. The connection between switch 1 and switch 2 is set to be in Trunk-Mode, Native VLAN is VLAN1 and Allowed VLANS is "all".
The RUT950 is connected on a LAN-port to port y on switch 2, setting needs to be Access and VLAN2, just like port x on switch 1. THE RUT950 needs to have a static IP within the range of VLAN2 on the used LAN-port, no DHCP. The failover WAN-setting on the Firewalla need to point at the RUT950's LAN-IP.
There is another option to connect the Firewalla and the RUT950 directly to each other, when you don't have a cable that long.
"Ethernet over Power" I was using that quite some time to connect my router on 2nd floor to my ISP's modem on 1st floor. To make that connection "kinda" secure you will need to set a special subnet on the RUT950, one that only allows 2 usable ip-addresses. One for the RUT950 and one for the Firewalla.
A word of warning: Ethernet over Power can be hacked! I was able to access my neighbors LAN, who was using EoP with a regular 24-subnet and DHCP enabled. You have to limit the ip-range and disable DHCP on it.
As example: 172.31.255.8/30 (255.255.255.252) usable addresses are 172.31.255.9 and 172.31.255.10.
I am just telling you what worked for me in the past. I don't recommend any product to buy nor can I say, if this will work for you.