A strange tale of Monterey installation, data recovery, and target disk mode

This was an interesting experience, and I learned some things. Hopefully others can learn from it as well. Spoiler: it has a happy ending.

My wife initiated a Monterey upgrade on her 2018 Macbook Air. It hung partway through. She was able to then boot into recovery mode, but not accomplish anything useful from there. I was called in to help. So in recovery mode, I attempted "Reinstall Monterey". Once again, it hung before completing. So I went back into recovery mode and now it said "There are no users on this volume to recover". Well that didn't seem good. I also tried internet recovery, no joy.

At this point I was afraid of touching the machine any further and possibly losing all the data. Did I mention no backup was done before attempting this upgrade? Also, it turned out that it had been a good long while since the last backup. That said, there was not much critical, irreplaceable data on the machine, so losing it all wouldn't have been catastrophic, but it was certainly not desirable.

Anyway: at this point we made a Genius bar appointment for my wife for the next day. Hardware diagnostics showed the machine was fine, but the Apple guy could not recover the data, and didn't know if there was any data to recover. He suggested a nearby shop that did data recovery, but no guarantees especially since it's a T2-equipped Mac.

The data recovery shop said they couldn't get anything off it, but that we should try the Best Buy Geek Squad. I confess to a very high degree of skepticism, but at this point anything was worth a try. The Geek Squad guy took the machine, and then brought it back out a few minutes later booted to the recovery mode screen (again, "There are no users...") and said the data is gone, nothing they could do.

Feeling almost but not quite resigned to needing to wipe the machine, I remembered one other thing I wanted to try. While fiddling around with safe mode and recovery mode and everything I looked at Startup Disk app and saw the option to boot in Target Disk mode, using a TB3 cable. I didn't have one of those, so before I left the Best Buy mall I grabbed one from the Apple Store and figured I'd give it a shot.

So I got home, booted the Air into Target Disk mode, and hooked it up to my iMac. Many seconds later, the Air drive mounted on my iMac... and all data was there and intact. I dumped it all onto an SSD, wiped the machine and reinstalled Monterey successfully, and restored the data. Nothing lost.

Here is why this is crazy:

1) Why did the Mac say "There are no users to recover" when, apparently, the entire contents of the drive were fine?
2) How did neither Apple, nor a dedicated data recovery business, nor Geek Squad think to try Target Disk Mode? I am still blown away by this. I really just tried it on a lark, figuring for sure that if the data were there then Apple would have gotten it. But no.
3) I did some Googling at the beginning of this adventure, and none of discussions I saw regarding this problem mentioned data recovery via Target Disk mode.

And so, I shout from the rooftop: ALWAYS TRY TARGET DISK MODE BEFORE YOU GIVE UP AND WIPE YOUR MACHINE. I don't assume it'll always work, but here at least is one instance where it saved our bacon. The machine has been running fine since then.

Hope someone finds this useful.