Thanks guys for the responses. Will update after calling DJI on Monday. Also have the flight logs so hope we can know what went wrong. I think the motor failure and ESC fault was a product of the crash landing and not the cause so will wait and see if the data will solve the mystery.
You can't. The only way to be sure that doesn't happen is not to fly
You can just mitigate them a tad on the few factors you can actually act on - like the props precisely for example, but 99% of what makes a multi fly is out of your control.
Even a hex/octo only offers redundancy in a very limited set of failure scenarios. A failing motor/ESC often takes out the others with it.