I'll even go so far as to agree a sear failure, while in the holster, shouldn't cause most guns to discharge - assuming it was in a safe condition to begin with. A cocked and locked 1911 won't discharge even if the sear vaporizes, and a gun with a firing pin block shouldn't discharge if the hammer or striker falls, assuming the gun is in a non-firing state in the holster.
I'd just recommend against horizontal shoulder holsters because of what the muzzle sees.
Did I offend? Cut me some slack, I didn't even mention Glocks in horizontal shoulder holsters. Or the danger of Glocks in shoulder holsters while practicing palm-in-armpit duck calls - https://www.youtube.com/watch?v=JuHMQ0QpH44
