So, because NASA had two catastrophic failures SpaceX is cutting corners? Nice try. Seats don't need to be "Space rated". Many internal components don't need to be "space rated" as long as the external components protect them sufficiently by being "space rated". You're barking up the wrong tree.
As for the strut, do you really think there are no components that slip through the cracks at the "big boys"? Have you ever actually built anything? Infant mortality happens; it's a fact of life. That doesn't mean your supply chain is crap though it is ONE possibility. There are many many other possibilities though.
With respect to your engine failure example, if you really believe nine engines are "required" to do the job, you're clearly wrong as they have successfully delivered payloads with less. They have designed in redundancy, which is NOT a failure, no matter how you look at it. "Main engine" or not, if the mission was accomplished safely, it IS success.
Is SpaceX cutting corners? Sure. That's how they have had the success they have. Are they doing it in a controlled and well measured manner based upon years of engineering experience (in and out of space)? I believe so. Their track record is pretty strong so far. Consider that the Space Shuttle program had 135 missions over the course of 30 years. SpaceX has had 19 in how many years? 5? According to Wikipedia, if they keep on track, they'll have something like 60 missions before they hit 10 years of operation at a significantly reduced cost.