The big question in my mind is what did the airline do for the UM after arrival. A 16 hour layover means an overnight. Did the airline find accommodation? Were meals provided? Was the UM chaperoned? And was the adult taking delivery at the destination kept informed? (That's more important than keeping the parents at origin informed, though that would have been courteous.)
If all that was done, then it seems to me that this is exactly what the UM service is designed for where there's a long delay.
I would have thought that the airline wouldn't readily have thought it worthwhile to do all this, than to re-route the UM, on other airlines if necessary, to ensure a same day delivery at destination. I wonder what prevented that?