Quick update - i followed @johnnie.black advice:
I backed up the /boot drive, deleted everything from it, downloaded the latest unRAID server: https://lime-technology.com/download/, and then copied over my key and super.dat file from my backup (/boot/config/super.dat and /boot/config/Basic.key).
Upon restart, the array started up successfully!
However (obviously), now i have none of my dockers or other system configuration. I'll spend a little time seeing if i can add back config slowly enough to find the culprit.