Skerit
You have said that your core is setup with internal and external on the same subnet. So the core's DHCP server also needs to be giving out IP addresses on that same subnet.
Therefore, nothing needs to "go through" the core to get to your "internal" machines as they are effectively simultaneously on both the internal and external subnets. This means that your so-called internal machines are perfectly routeable from your physical broadband router - the reason you are having problems is because you are trying to force your traffic from the broadband router to the core and then back to your internal network - but the internal network is the same subnet as your external network, so it isn't possible to route that way (or at least it would be complex to set up, using route metrics).
But more importantly, there is no point in doing this anyway. If you are trying to get external traffic to a machine that is on your "internal" network (eg you are trying to publish a web site), just set up a NAT on your router directly to the machine's IP address. It doesn't matter that the core thinks that this is "internal" they are all actually on the same switching domain and subnet, so the traffic will go directly from your broadband router to the machine. The only problem you will have is that the address is dynamic from the core, but you would have this problem even if you were sending the traffic through your core. On the upside, once LMCE has assigned an IP address to your machine, it will pretty much stay the same indefinitely as it recognises the machine by its MAC address and assigns the same one every time.
Having internal and external on the same subnet and effectively using the same NIC does work with LMCE (I had it running for a while in a single NIC installation) but I think you will find it confusing, and LMCE certainly does - it discovers itself via the DHCP pnp process and presents its own internal HDD as a remote network share to you (called DCEROUTER) you just need to tell it to ignore that server.
You would find it much more straight forward to set it up as 2 separate physical networks with the correct separate subnets, and then just buy a wireless AP for the internal network to get around your cabling issue.
BTW - when in the "correct" (as designed) config, you will need you firewall turned on in many situations as most broadband routers cannot NAT (port forward, virtual server, and a number of other names!) to remote subnets. So you need to NAT/port-forward to the external IP address of the Core, then set up a port-forward rule on the Core to send the traffic to the backend - this works very well and is easy to set up. If you were just routing (ie not NATting/port-forwarding from the Internet) then Linux will happily do this on its own, without the firewall function. But if you do leave the firewall on, the you will have to create a normal firewall rule instead, to allow the traffic through.
What traffic are you actually trying to get through??
