Hope that helps..

O.K. Surprisingly enough I understand what you are saying, however I still cant ping the external router from the internal NT clients.

Throughput stops @ the linux router in both directions.
I can ping the internal interface (192.168.111.10) from the external router.
I can ping the external interface (192.168.15.2) from the NT clients.
I cant ping the internal interface(192.168.15.1) of the external router from the NT clients.
I cant ping the NT clients from the external router.

I would run tcpdump on your router box and figure out what is happening to the traffic.

tcpdump -vv src net 192.168.111.0/24 and dst net 192.168.15.0/24 > ~/capture.txt

That should capture data going from the one network to the other and save the output in a file called capture.txt in your home directory.

then ping from a client in 192.168.111.0 subnet to the external router at 192.168.15.1 to gather some data.

and post the output of route -n here as well

O.K. Here's the deal:

Everything is working now, and without NAT as intended.

You're advice was correct from the start. Routing was actually working the whole time. The problem was that the firewall was also working. It seems as though that the iptables service in Fedora is much like the ICS/IF service in Windows NT 5.x. When the firewall is disabled the service is actually still running, and obviously if there are no rules then there is no traffic allowed. I took iptables out of runlevel 5, and routing immediately began working as well as the NAT from the external router. My problem with routing in the past had been my failure to add a route to the internal LAN on the external routers. I presume I was always sending traffic to the external router, but it was never able to make it's way back.

Thanks to your specific help I was able to determine where the gap was and that helped me narrow it down. You were specific enough to where I will be able to apply what I learned from you to every network in the future. Thanks a million. I'm not sure how the n00b can help the master but I owe you one.

Now comes the nightmare of learning how to write custom firewall rules on an OS who's syntax I don't quite understand yet..

I think I feel comfortable enough to switch to a lower-scale, less user-friendly distro as well now. Any suggestions from anyone?

For anyone else out there with similar problems:

1. Network Manager is a common reason for boot-activation failure in dual-NIC scenarios. If you can activate after all your services are started, but not during boot, check your log files thoroughly line by line.
2. Configure all routers for all networks they are not directly connected to.
3. IPtables may still block traffic when "disabled".

2 useful iptables commands for troubleshooting.

iptables -L
and
iptables -F

Glad you got it all worked out.