...so um.
It lives!
I have no idea what happened. I erm. Borrowed a power adapter from work, plugged it in and...nothing. I fiddled and prodded and pushed and pulled, but nothing.
So I went to work on switching back to just the SH2, reconfigured everything, got frustrated at the arbitrary limitations of the SH2 but sorted it. Then as I went to move something, noticed a couple of blue lights sheepishly blinking at me. It was alive.
I plugged the old adapter back in and again it was alive. Off and on no problem, as if nothing had happened.
I was more worried about losing my IP than anything else.