I'll be honest and state I've only used unRAID for about 78 hours so I am not able to answer your question. If no-one responds in a few hours I'll dig into this after work today and try to assist as I also need to know this soon.
Regarding the NIC assignment; One thing I have found interesting during my brief trial period of unRAID is that once I set the "Static IP" within the web interface(didnt even think to check which NIC this was setting -_-), I was able to take my thumb drive from an Elitebook 840 G2, to an r710, to an HP 8300 SFF, to a custom built PC, then back to the r710 WITHOUT EVER EXPERIENCING AN IP CHANGE EVEN ONCE.
All of the above systems had at least 3 NICs available for unRAID to choose from, and it always selected the NIC that had an active ethernet connection and used the IP I had set via webgui. I know this doesn't answer your question, but it makes me wonder if there is even a "standard" way to declare the primary interface and it's associated configuration, given the results of my experiments this past weekend.