Was there anything remarkable about my aborted return flight to Chicago from Miami, in having the preflight check reveal non-working flaps so the failure occurred while the plane was on the ground, and not during flight?

This was indeed a divine intervention and it happened because you did prayer work asking for a safe return home and were guided to do so by your higher self. This was an accident waiting to happen and the sequence of events was simply to accelerate the failure so it would take place during the preflight check and not during flight. This could have resulted in a crash and would have been quite devastating for many onboard. In this instance, the overriding needs of karma were not such that innocent lives needed to be risked, so your prayer was highly effective to enable the divine realm to forestall the event.