Cannot mount ZFS pool due to "insufficient replicas"

[HoneyBadger]

I'm actually more relieved to see that you hadn't uncovered some bizarre behavior in ZFS, and that it was a missing disk. A simple problem with a simple solution, and the insistence probably made you go take that second look. No harm done, and no sleep lost (at least on my part; hopefully you'll sleep better now with your kid's Minecraft worlds restored!)

If I may ask another question. I am using ZFS to give me resilience, but this instance has shown that I do not have that on this storage pool. I am assuming that I should be using raidz1 to provide the resilience I want, e.g. to be able to loose a disk. I am still a bit confused as to why a mirror would work like it has here.

As @Samuel Tai mentioned, the best solution is to either configure a 3-way mirror (if you want more redundancy) or the RAIDZ1 (if you want more capacity) - with 2T disks it's not a terrifically perilous setup. although Routine backups of the kid's Minecraft worlds to another dataset if you go with Z1 wouldn't be a bad idea though.