In short, "Hard Drive - Not Installed" kept appearing. Booting --> F12 --> Diagnostics confirmed the original SSD drive was absent (whether it was in the slot or not). Meaning, the SSD died.
Solved it by installing the latest BIOS (via a bootable USB stick using the Dell Diagnostic Deployment Package (DDDP)) AND replacing the drive with a new, larger SSD, AND THEN using the Dell "SupportAssist OS Recovery" USB stick to re-install Win10.
Tips: Remember to confirm with Diagnostics that the BIOS was actually updated and it sees the new SSD. The .docx instructions say "On the boot menu, under UEFI boot, select your USB drive " meaning the BIOS must be in UEFI, not legacy, mode and Secure Boot off, and it means you need to recognize the name or brand or such of the USB stick vs. the new SSD... and don't pick the generic 'USB drive' option under Legacy boot choices. (That itself was an hour lost.)
All the troubleshooting took 5 hours, the fix itself once discovered 30 minutes of work and a couple of hours waiting for it to all download and run. All data on the HD was lost, of course, but the laptop works again.
DELL XPS 15 9550 Hard Drive - Not Installed
For my notes, Click my name and private message me the pc service tag number as well as your registered name, email address, and phone number.
If your computer is out of warranty click the link below.
You can try turning the computer on and tapping the F2 key to enter the bios. Once in the bios choose the option to Restore Defaults. Save the changes and exit.