i fixed the unhandled exception by upgrading to 9.10, re-imported my backup pool and am now restoring from that over the LAN
according to zpool detach, I shouldnt even be able to detach the last device in my pool.. but it did anyway and nuked the pool.. in any case, I nuke zpool's on a regular basis with mistakes and this is a good chance for my backup solution to be fully tested, haha.
I suspect my back-up box's nginx error was due to an old USB boot device, I'm waiting on my intel x-25M's from ebay to replace that.. it was probably not an issue with freenas 9.3 - I kept having to re-configure the network interface, it was never getting saved - probably a sign of the usb drive going bad.
The 9.10 detach issue mystifies me.. I just learned long ago not to f with pools once you make em, and broke my cardinal rule after getting my backup box working.. I've re-learned my lesson and will not touch pools again.
It's like 50-50 getting em to re-import even with all key's and sh*t saved lol, not because of FN, but because I neglect my setups and let them get 2 year out of date and some bug that was fixed a year ago pwns me.