When I initially loaded the backup, the editor was still choking. At first I thought the backup was corrupted somehow, but it passed JSON validation checks. So, I considered that I had a corrupted SD card, so I replicated the SD card, the copy seemed fine, until the bad flow started, but at the time I did not realize the bad flow was an endless loop. So I then created a clean OS image, a clean install of node-red, and I still had issues with backup flow file. Of course, it was not until I ensured that the specific flow that had the endless loop, did not execute (removed it from the flows file via edit) that my clean OS, clean node-red instance was stable.
The original flow file, did in fact disappear from the file system, when I looked for it right after node-red went unresponsive, editor timed out, dashboard lost connection, etc. I did not power off, but did make sure that node-red process was completely dead, stopped. I was looking for the file, and copied the backup file to be the active flow file, while node-red was completely stopped. It was only when I decided the SD card might be corrupted or failed, that I powered down the device.
I had not thought to try to recreate the exact scenario, since I believe I know what had happened. I did create an endless loop yesterday in error, while reworking the broken flow. But I was careful to explicitly stop node red before it crashed. Stopping node red while it was struggling to complete a deployment, during the endless loop, I did not see the flow file disappear again.