Unplug your TV?
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Certainly.
The Upgrade command is to just ensure all packages are up to date. Whether Hes is ready to run autoremove I leave entirely at his discretion...
Can you boot into the Recovery Menu - drop to Prompt and hit enter, then run:
sudo apt-get install xserver-xorg-input-all
I turned it off and did a hard shut down. No difference. 
While we are flailing, you might also try:
sudo apt install xserver-xorg-video-all
Sometimes power off the monitor is not enough. You need to physically disconnect it. (From my Raspberry pi experience).
Since his mouse and keyboard are not working on login; this tells me that the installation of xserver-xorg was the first installation, so input-all is a needed package to restore the mouse and keyboard functionality.
In the meantime... this is a really bad time for me to run away- but I actually have been lingering and am running late for an appointment...
Hopefully the installation of input-all will get Hes able to log in and operate normally for now and you and others can continue to help troubleshoot the flickering screen issue.
Unfortunately, it is midnight here in Paris and my concentration is at the limit. I can certainly continue on when I get up tomorrow morning.
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Thank you so much for your help. I am pretty stuck right now but will see if I can get to a workable point while you both are away. No need to devote any more time if you don't hear from me that I got back to some decent point.
Sometimes not hearing back means the computer is a mess, too 
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Long before internet, mobile phones and tablets, I quickly realized that I needed a computer to fix another computer. At each upgrade, I assembled a sub-machine from the leftover parts - which was awfully looked like my previous machine 
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