Thanks Magic Banana for taking so much of your time for a problem that seems
unsolvable.
I did try the Ctrl+Alt+F1 method to get a terminal but once the system has
booted and I get a graphical interface, not one key would answer.
But during the init process, mesing with the keyboard did display the
"verbose" of the process. And the first thing it read was about not finding
the i8042 controller. Duckduckling this message, I found out that it has to
do... with th keyboard - which makes sense. I even got a solution here
(someone who's also installed an Ubuntu-flavoured system on a Macbook) :
http://askubuntu.com/questions/655719/i8042-no-controller-found-error-while-trying-to-install-ubuntu-on-mac-book-pro
But still, I'd need access to either the grub edit option or to a terminal,
so to no avoid.
