I had a very similar situation like you
my SLI stopped work but single gpu mode worked, got fed up and upgraded to a 780ti
I gave the second SLI'd 580 to my cousin as is
works as a primary as if fresh from the box
is the stripped screw really your fault?
I've taken apart cards to install a backplate and the screw was already stripped when I took it out (luckily they gave me extra screws with the backplate so I don't have to use my originals)
take the screw to the hardware store or perhaps an electronics store and see if u can get a replacement of the same size and threads