If you can fix the bolt, then this solution worked well for me.
I too had peeling backing and cracks.  A window shop shattered
the mirror when trying to remove it, so they cut one just a bit
smaller and simply stuck it in with copious gobs of silcon, then
ran silicon around the edges to keep out water.  It was a theoretical
solution, but it has survived two Michigan winters so far....  Total
cost, I forget, but I was pleasantly stunned at how cheap it was.

good luck
=========
Steve