Which plot development would most logically lead a reader to conclude that the themes of this story are forgiving oneself and others and embracing new beginnings ?

The best option would be, In my opinion, option c) which states that they and their spouses can all become friends and joke that their children cannot date each other.
If they have agreed that they and their spouses can be friends , then it means that they have forgiven each other and themselves. As for the second part, the most important term here is ‘jokingly’, which means that they did not mean it to be an iron-clad rule and it may change in the future.