Unable to boot to recovery mode after problematic High Sierra upgrade
When macOS' recovery partition fails, it creates a Catch-22 situation where you need the patient to be able to be able to perform brain surgery on itself.

If you have a ≥ 8gb usb drive you can erase handy, you can use it to create a recovery drive which you can use to diagnose and (one hopes) repair your intractable Mac drive(s).  It'll have all the same tools recovery mode offers:  you can test, run first aid, or perform other related functions without being dependent on the disk you're trying to repair.


Choose  > App Store… and download the High Sierra installer if /Applications/Install macOS High Sierra doesn't already exist.
When the download completes, the installer will open automatically.  Quit it.
Edit the following command, removing any returns to make one long line.  Replace YourDriveName with your USB drive name.  

sudo "/Applications/Install macOS High Sierra.app/Contents/Resources/createinstallmedia" --volume "/Volumes/YourDriveName" --applicationpath "/Applications/Install macOS High Sierra.app" --nointeraction
Open /Applications/Utilities/Terminal.
Copy/paste the above command at the terminal prompt, press return, and enter your password.
After the process completes (it'll take awhile - be patient), restart your computer and immediately press and hold option.  This will eventually display the startup disk picker, from which you can choose the recovery drive you created rather than your normal start disk.  That'll at least give you a reliable work space unaffected by whatever's wrong with your drive.