2011 iMac Panic High Sierra

Mid iMac 12,2 27" High Sierra had been randomly booting to gray screen. Now not booting to OS at all.

Runs perfectly on Linux Mint Live CD so I assume the hardware is fine??

Option at startup shows Macintosh 250 GB HD (boot SSD) and Macintosh 1TB HD2 (storage) as well as EFI.

When selected, none complete progress bar to boot.

D selected at boot and run extended diagnostics shows no hardware issues.

Command+R completes but then goes to gray screen.

Never brings up the Utilities screen.

When still somewhat functioning, I transferred needed pics and documents from the boot SSD to external HD. HD2 still contains lots of pics I want to save.

I also have a late 2009 iMac (High Sierra) and an 800 firewire to 800 firewire on order to hopefully transfer HD2 contents via Target mode.

Once completed, I would like to reinstall High Sierra on the 2011 iMac.

***Is it possible to wipe the 2011 HD and install High Sierra from the working 2009 over the firewire to the farked 2011?

***If that fails and I am able to install Linux Mint to the Mac SSD, wiping and reformatting the drive, is there a road back to MacOS? How to recreate APFS on the SSD?

This mac will be going back to a friend who is all about Mac and would want Mac OS.