I recently installed Zorin OS 16.3 Lite on my iMac 5,1 and the "safe graphics" option in the installer worked (after 5 minutes of a black screen which was still on ("fake" black.)) When the installation completed, I rebooted the iMac, and after some time, got into the GRUB menu! The "Zorin" option booted to the fake black screen, and Control Option/Alt F1-F8 did nothing (though maybe I should be concerned that Control X did not boot Zorin, only F10.) Changing ro quiet splash to nomodeset quiet splash and just nomodeset did not work. I have been considering connecting an external monitor.
Thanks,
acuccoder