What is the probability that a brand new but defective part will cause a catastrophe? How can that be mitigated to prevent catastrophe, and what is the new probability assocated with that built-in redundancy?

What is the probability that a well-used and proven part will wear out within a specified timespan? And again, how can that failure be mitigated?

This is called Failure Mode Effects Analysis (FMEA) and is based on statistics. Every part on their vehicle has to be tested to destruction, numerous times, to determine statistical operational lifetimes, failure modes, and necessary redundancies. It is very difficult to do this at design time without substantial a priori knowledge of components being used.

My impressions of Spaceship One test flights was that there was little redundancy. And the legal and legislative problems that could arise after a fatal crash, not to mention the possible drop in customers, I think are significant. This is a very high risk business venture, nothing like selling everyday products like toothpaste or macaroni & cheese.