Don’t forget unscheduled and charter airlines. They were commonly used during the day...the first Kenya airlift was a charter that cost 81 passengers about $330 each it went to New York.
I would not put it past someone going to Hawaii to go by a southern air route bypassing the mainland USA, east from Kenya to Hawaii. Also for one going to Hawaii, ship transport is also likely in 1959. If one had the time it could be done on the cheap via cargo ship or working their way across for free.
Thus explaining why a very very very pregnant Stanley Ann was not in Mobassa, she was trying to catch a flight back to the US to have her baby at home, something she couldn’t do from Mombasa.
Assuming that she actually did have the baby in Nairobi, as witnessed and reported by the relatives, of course.