To everyone who posted ion this thread (JoseMR in particular):
Thanks so much for all your help. I was able to do as you suggested, and determined that there was nothing in the /data folder on the original USB stick. It appears that it was truly borked.
At that point, I just fresh-installed FreeNAS 9.3, which was the newest version I had on hand that would work with the hardware in place, and set up everything from scratch. It was a real hassle to configure users, permissions and such (since I don't know the system) and I wasn't able to import the pool, since I only had a single drive. I ended up clearing the drive and having the client restore everything from their backups. Not ideal, but at least they're back up and running.
I never could have done this without the help of this community, and JoseMR specifically! It's all the more amazing to me, since this isn't even your software!
Interestingly, I still haven't received a reply to my latest post on the iX forums.
This experience has inclined me to think about switching my Windows server at home over to a XigmaNAS-based system, when it comes time to replace it. Thanks again to those who commented and held my hand through this ordeal. I'm sure many of you could have solved this issue in 20 minutes flat, and maybe even preserved the original data, but coming from zero FreeBSD knowledge, I feel happy to have accomplished this much, with your (considerable) help.