The Question and Answer section for The Tempest is a great resource to ask questions, find answers, and discuss the novel.
The mischance of the hour refers to the possibility that the ship might sink in the storm. The Boatswain says the king's advisor might council the wind and the sea to stop battering their ship. He is of course being facetious.
The happiest event for Alonso is finding out his son is still alive. The second happy event would be Ferdinand's offical engagement to Miranda.
Prospero demands that Miranda remain a virgin until her marriage.