The conditions of carriage say nothing about Cathay needing to offer you the best route to maximise your AA miles. When their flight was delayed they found you an appropriate alternative to get you to your destination as quickly as possible. You declined it. At that point anything else Cathay did was pure goodwill on their part - they could have just left you in Singapore since you declined equivalent travel at their expense.
Thankfully, the American lawyer-driven compensation culture is limited essentially to America. It doesn't get you very far elsewhere.