2.Please explain the conceptual difference between "mutuality” and "reciprocity.” Why does English think that friendship is the correct foundation for filial obligation?

The conceptual difference between "mutuality" and "reciprocity" is that  "mutuality" values the relationship of people. People are motivated by love and they do not want to gain any extra benefits from others.
However, "reciprocity" considers about the quantity of benefits. People would care about whether the action is equal and fair for them and others.
In the essay, it has mentioned that "Friends are motivated by love rather than by the prospect of repayment." English thinks that, for friendship, people would not only care about the "return" in the future. They would behave because of the relationship of love. Using this way for filial obligation can maintain the relationship by showing the love. However, if we replace the idea of duty instead of using friendship for filial obligation, it will not exist for long time as we would count the quantity of "return". The relationship may be destroyed easily at last. Therefore, friendship is the correct foundation for filial obligation.


 3.Do you agree with English? Do you think that friendship provides an appropriate explanation for the moral relation between adult children and their parents?

To a large extent, we agree with English. What parents do for their children are voluntary sacrifices, which is an expression of love. Therefore, when their children grow up, the reason of them  treating their parents with filial respect is not  repaying them but to show their love. Just like friendship, friends will do each other a favor without measuring. 
However, friendship cannot totally explain the parent-child relationship. Since friends share the same position but parents and children are not at the same position in the family hierarchy. Children should always listen to their parents, which is different from friendship.