Hyper-V Cannot load a virtual machine configuration

Well, this is a strange means.

The key thing is Windows/Hyper-V is referring to a "list" somewhere to tell it what virtual machines are registered with the server. My virtual machines are scattered across multiple drives, so there has to be a centralised repository. Which turns out to be

C:\ProgramData\Microsoft\Windows\Hyper-V\Virtual Machines

there are symbolic links that refer to the physical locations of the actual config XML files. I noticed the troubled VM links had a "lock" icon.

I edited the security ACL of the troubled symbolic link. Sure enough it does not have the NT Virtual Machine account like the rest, so instead I granted it Full Control to the Users group. Restarted Hyper-V Virtual Machine Management service, and it could once again load up the missing VMs. It appears to work without the Virtual machine accounts.

Still haven't gotten the complete deep answers I was looking for to explain just what Hyper-V is requiring out of those Virtual machine accounts, but at least the original virtual machine config can be reused.