Wow that is incredible it survived so well. I think you got a 1 in a million impact on the most impact resistant part... that leg must have taken 80 of all the force and cusioned the body when it broke. It has to be said they have their faults but DJI have made a tough cookie!
Glad you were able to recover it (especially with the help of a fellow pilot!).
I think you are right about the prop coming off rather than the seize up... the fact the prop nut is gone says it all as if that was ripped off in the crash the prop and shell would be shattered everywhere. I have those CF props and i find I need to tighten them every flight just to be sure. Nylock nuts next time?
You probably have done already but I'd bin those props just incase too... Carbon is a strong but brittle material and with the forces involved in flying a hairline crack could shatter the prop mid air.
Nice to see a happy end to such an event!