This is how I presume it is now:
 Code:
Local Lan -> FireWall -> Internet
                  |
                  V
               Cisco Router -> Remote Lan

I would suggest to use the first "code" you wrote, Gargoyle
I presume the Cisco Router is 192.168.24.2 and the firewall is 192.168.24.1
I would try to set the Cisco as the default gateway via DHCP
The routes on the Cisco would look like this
 Code:
ip route 192.168.100.0 255.255.255.0 "NextHop"
ip route 172.16.4.0 255.255.255.0 "NextHop"
ip route 0.0.0.0 0.0.0.0 192.168.24.1