Maybe this sort of thing, lets call the devices:
a) "Front pfSense", which has OFFSITELAN as a VLAN, and WAN and WAN2.
b) "Back pfSense" which has its WAN on Front pfSense LAN. And Back pfSense LAN has some ordinary local users also.
Front pfSense
a) add a gateway to Back pfSense WAN IP - "BackGW". Probably no need for a route.
b) On OFFSITELAN add a rule, pass source OFFSITELANnet, destination any, gateway "BackGW"
Back pfSense
a) On WAN add pass rule to allow source OFFSITELAN subnet.
b) Firewall->NAT Outbound, go to manual and add NAT rule on WAN for source OFFSITELAN, destination any, NAT to WAN address - this will ensure that packets from OFFSITELAN get their source address changed to the Back pfSense WAN IP, and so returning packets will have to be routed by Front pfSense to Back pfSense, and unNATed to deliver again to Front pfSense, which will deliver to the OFFSITELAN client.
That should all work without any Squid. Then add Squid to the equation and see what happens!