4. By " 'system' stage " do you mean the word 'system' shows on the screen briefly while attempting to connect in preboot?

YES. The words 'system' and then the word 'Loading' shows on the screen briefly while attempting to connect in preboot

5.3 Device Manager where it should show as "GARMIN Device" even briefly if it does connect in preboot mode

YES. I had not noticed that. I heard the laptop 'beep' sound and Dev Manager update so I knew the usb port had seen the device for a tiny few moments.
I do see Dev Manager showing "GARMIN Device" briefly while trying to connect in preboot mode ( while 'system' and 'loading' displayed above 'Garmin' .

5.4 Does it show anything else on the screen before leaving 'Loader' message then powering off? Like a calibration screen with white dots and a grid?

NO
It goes back to ''Garmin' , garmin disappears , 'GARMIN' again and so on until i disconnect usb cable and power button off.

7. If it is showing in Device Manager briefly as "GARMIN Device", it will also show in G7ToWin but will be held in preboot once G7ToWin properly recognizes it and exchanges data packets with it. Similarly to it being held in Updater.exe only after a flash is initiated.

I will try G7ToWin again tomorrow, so far no success, need a bit more time to learn how that works when i select usb interface.

8. YES
I was able to generate the GarminCure3 firmware "226701000590.rgn". This running Cure3 as an administrator. It created the *.rgn in the 'GarminCure3-40F78E54' sub folder .
Updater.exe runs too . I set it to use usb.
After about 5 tries I did not get to where Updater.exe acknowledged a usb connection while I was in the Garmin Satnav 'system' and 'loading' boot phases.
I will try this last step again some more tomorrow and update you.

Tks for help .. very interesting