Well, that idea solved the screen saver issue, but not the blank or flickering screen upon logging out of mate. I did notice when installing nvidia-driver that the file name had changed from 470.xx to a 500.xx. Thus, my assumption is that the updated nvidia software may have added additional issues such as a need for a second video path for dual monitors, or some kind of driver for some sound volumes or such. I'm not knowledgeable nor experienced enough to solving such things. Therefore, any additional thoughts will be very much appreciated. It maybe my MSI motherboard BIOS is causing this, but until about a month ago everything was running fine. I'm an avid user of "less /usr/ports/UPDATING and have not found any relevant upgrades therein.
Oh! now upon logging out, the screen goes blank and I have to type, in root, shutdown -p now without seeing anything. But, if I've typed it correctly, the system shuts down.