Started working reliably again eventually....

I haven't tried running it in a VM; it will probably work, but are you aware of the general restrictions of running Mac OS in a VM? I.e. app store apps don't work, can't log in to iCloud, etc. As you have plenty of disk space, have you considered dual-boot? This does work, though currently you have to install the current Mac OS, boot into it, and then upgrade to the beta. There is another thread about that, in which I reported lots of difficulties until I discovered that an external drive attached to an M1 Mac Mini is not bootable connected to USB-C but is bootable connected to USB-A.

Invent another son who is a few years older?

How can I speak to a real person to discuss this? If it's actually an Apple security vulnerability, you could try their security reporting program; see https://support.apple.com/en-us/HT201220 Other than that I would not expect to get any useful interaction with Apple. "Report a bug, and post the number here, thanks." Have you considered talking to a journalist?

Are you also getting the location?

The "best practice" is NOT to have your clients share your developer account. They should have their own accounts with Apple, and make you a member of their "team".

Is the file UTF-16 or something?

It's likely that you'll need to permit BOTH booting from externals AND relaxed verification. On my M1 Mac mini, there is no setting to enable/disable booting from external drives - I've seen the switch in screenshots elsewhere, but I don't seem to have it. I do have the switch for "relaxed verification" but I don't believe I needed to relax it to make it work.

This version can be identified by the presence of the word JazzKnob. Grep can't find this text. What exactly happens when you try?

FB12301774

Finally working!

This does seem to be resolved in the macOS 14 beta.

I am making progress at last. I found a clue in a post on the macrumors forum: you need to connect your external drive to a USB-A port, not a USB-C port, at least on some Macs. (Mine is an original M1 mini.) Having done that, it installed Ventura and booted into it with no trouble at all. It is downloading the Sonora beta as I type this. "About 1 hour remaining".

I have installed macOS Sonoma on a separate volume. Created a separate volume (https://support.apple.com/en-gb/HT208891). Created a bootable installer for macOS Ventura using https://support.apple.com/en-gb/HT201372. Install on macOS Ventura on new volume. Upgraded to macOS Sonoma. I'm failing to make this work on an external drive; I can't get it to boot Ventura from it. Is it supposed to be possible to do this? I've tried two approaches. Firstly, I've tried to run the macOS Ventura installer and told it to install to the external drive. I then restart holding down the power button and am presented with the choice of disks to boot from, but when I choose the external drive with the new installation and click "continue", I just see a spinner for about a second and then nothing happens. The second approach I've tried is using the createinstallmedia tool. I've partitioned the external drive using the GUID partition table with a small "Mac OS Extended" partition and a large APFS partition. I've then used createinstallmedia to put the installer on the small partition, and booted into it, which works. I then use that to install macOS onto the larger partition, which seems to work, but subsequently when I try to boot into that, I get the same behaviour as the first approach i.e. it spins for a second and then does nothing. I note that the "Use more than one version of macOS on a Mac" page that you linked in your point 1 is specific about "a new volume of your startup disk".

The status of my bug report is now "Potential fix identified - For a future OS update". I guess that is an acknowledgement that it is a macOS bug, not something I'm doing wrong! Hopefully it will be resolved in a 13.x update and my users won't have to wait for macOS 14 to be released.