Ed,
Thanks for finally posting. I've been thinking about your problem and I  
believe I know why the piece of truss snapped. In general trusses are made from  
oriented strand plywood. I believe that if you had cut the truss differently,  
say 45 degrees different, the grain would have been different and you wouldn't 
 have been able to break it.
I had some hair line cracks show up on my mount a couple of years ago and I  
added a stainless steel backing plate cut to the same shape as the mount for  
additional support. No problems with it since.
 
Rummy