So, I realize this was a while ago, but did you ever get to a final conclusion? The reason I ask is that I just did something kinda similar.
I had a breadboard setup working with a nano and moved it to a shield I got from Amazon:
I plugged a 12V DC transformer into it and nothing happened. I took it back over to my confuser and plugged the into the usb port on the nano and the magic smoke appeared. The nano appears to be fine but the shield has burn marks on the bottom.
Perhaps powering the nano via usb when it's plugged into the shield was a no-no that I just learned!
Thoughts?