Collin, dude, that was it! Ok here's my observations and (temporary) solution.
When installing the core - perhaps on my particular combination of hardware (and luck) - eth0 gets assigned to the internal network. I installed the whole thing twice and it did this consistently. As Collin commented on one of his previous posts, there's an error during the install or first boot that might be the root of the problem. I see in the firstboot.log a "device1 has a bad mac address" error then everything gets messed up from there, it seems.
I installed two new NICs on PCI slots for now. I wrote down the mac addresses and went to /etc/udev/rules.d/70-persistent-net.rules and edited eth0 for the corresponding mac address I wanted to make my external and the same for eth1, the internal card. Save, reboot and all is good. It connected to the mothership on the external net and discovered my HDHomeRun on the internal net.
After I get to play around with it a bit more - I haven't slept a wink in 26 hours now messing with this - I'm going to try the above solution for the internal cards. BTW, I installed this on a Gigabyte P45 motherboard that I heard elsewhere was not compatible with MCE. As soon as I get it a bit more stable, I'll add the case to the wiki.
Thanks all for your help, I guess we can close this one out.