Thanks for your input.
1: Yes, the proposed keyboard solution does work.
I dont' know why this approach did not work with apt-get install on my last attempts.
2: Yes, answer #7 of the solution for the guest-additions did work.
I did not know that I first had to add an empty optical drive in the virtual machine settings
And I don't know why I did not find these solution during my googling sessions. Sorry.