The initial problem, (the failed relay) that led to many other poor choices leading to the crash, I think is going to be a more common problem.
I experienced a master relay failure in flight as well. During normal gear extension just as the mains locked and the pressure in the system came up the pump pulled a few more amps and connection inside the relay failed. Thus, the plane went electrically dark. Luckily I had seen 3 green before it went out..
But the bitch of it was after landing I cycled the master switch a few times and bingo it came back to life. I couldn't reproduce the failure on the ground until I tried to start the airplane. As soon as I pulled high amps on the relay it quit and I was able to measure across the relay in its failed state. My relay was mounted below the battery as well but looked to be in good shape externally. I replaced it with a new relay found at NAPA auto parts. But, be sure to get a continuous duty relay, not a starter relay. They cost about twice as much, I assume due to the precious metal on the contacts.
Just FYI for those flying older airplanes.
Tom McNerney