Unless you want to change your shifters you are going to need to stick with Shimano.

As for the derailleur I had that happen once as well. My front tire kicked up a small stick that managed to wedge it's self between my derailleur and the ground. Had I not stopped imediately I would have never seen the stick.

I doubt that you are going to convince a shop to warranty this one. This is just part of the risk you take riding a bike off road. It's a risk I'm willing to take though.