That's very interesting and I really appreciate your interest and support, you've done more than anybody has since I got myself into this situation on the 20th of November.

I have two SSDs each with an individual (encrypted) APFS volume. One (labelled Radix) is the boot volume and ends up mounted on /. The other one (labelled Usor) is meant to mount on /Users, but on account of being encrypted, gets ignored by automount (despite having a valid /etc/fstab entry) and just lingers, until I log in into a utility account, allow it to decrypt and mount, log out, and log back in as my user. My user has the home folder set to /Volumes/Usor/User/james and then it all works. But as lament, half the stuff is broken.

As soon as I get back to the office I'll have a look at whether the ignore-permissions flag is set or not (I reckon it isn't, but it is worth checking). I've faced more path-blocking absurdities in this enterprise than I have in the past ten-plus years of Mac usership.

I have almost no experience with APFS. I did run ZFS on my Macbook which required have / and /Users (on ZFS). It worked, but the performance was terrible. Sending snapshots to another machine was magical.

I am much quicker to capitulate defeat in the face of unique setups like this. Or at least experiment with a VM and keep my work machine more main path. You will hit bugs, it looks like you already have.