Can you avoid both problems by booting rescue media and choosing
"boot from HD"
or "boot installed system"? If yes, it would seem to me to confirm bootloader
the root of the trouble.
I have made a live USB stick from KDE spin of F27. The live KDE works
well with normal screen resolution. However, I do not know how to
"boot from HD" or if I can use it as a rescue media. Could you tell