A tentative solution to my second problem, which will allow me to wait until my
"grub2" menu is working on both standard Linux startup and on Recovery (Recovery
failing now) : I just created a bootable USB key
with the "System" "Administration" "Start Disk Creator". This allows me to boot
my computer on this USB key, then using "sudo" apply modifications to the
failing file, for exemple xorg.conf, which does not allow a clean startup.