You can set one cluster member to be higher priority than the other such that the cluster will prefer to run on that member if it is healthy. I personally wouldn't, since it makes it easier to not notice problems with the other member until it's too late and you need it to work.
I would connect both telco routers to the switches so both firewall members have the same visibility of everything. A failure of the core switch would prevent the attached firewall member or telco router from being reached, but a failure of either telco or either firewall member shouldn't necessitate a failover of any other part of the infrastructure.
If you need to conserve IP addresses, you can use off-net member IPs so you don't have to burn an address for each member. That is, you can use 2.3.4.5 as a cluster VIP, and 192.168.144.121 and .122 as the member IPs on that interface. This involves adding a local interface route to each member telling it 2.3.4.4/30 (or whatever) is out the interface so the firewall knows to send ARP requests rather than looking for a gateway address.