Missing Firmware Partition after NVRAM reset in MacOS High Sierra
You Title is bit misleading for me.

Based on your description on what is going on lets try to repair your disk.

1- check the cable connections for firm fit.

2- Start or restart your Mac. As soon as you hear the startup tone, press and hold 


  Command-S


on the keyboard. Keep holding down those keys until you see a black screen with white lettering. This is called “booting into Single User Mode.” As soon as you see the black screen with white lettering, you can release the keys. 

Wait for the text to stop.

The last line should end in root#.

Right after the root# prompt, enter the following:

/sbin/fsck -fy


Press the Return key.

It will attempt to repair your disk and tell you the result.