I have tested my original "broken" mouse on a laptop and it seems it functioned quite alright. It leads me to think there must be something erring on my desktop apparatus. I will test the mouse once more perhaps tonight or this weekend.

Another thing remarkable is the peculiarity of the sudden switch to the Greek keyboard layout. It is actually very simple: as I type I occasionally hold/press [Ctrl Shift] to quickly select words with the arrow keys, and it happens to be that I set the combination of [Ctrl Shift] to switch language (-- it used to be [Shift Alt]). This explains the sudden unexpected switch, for that particular moment only however. I am uncertain whether this was the case before as well, as I paid no attention to it then, but it is plausibly the case too.

This still leaves the sudden appearance of letters I did not type nor were the keys pressed by anything else noticeable. Thus far the situation with the old PS/2 mouse is good, and the fact that my original one works fine on another computer certainly signifies -- as said -- trouble on this desktop. Yes yes, indeed, I will test.