Thank you for bringing up a topic which moves me.
Hmm ... I read most of the books you mentioned and am interested in the questions you named.

"We" (as humans) indeed figured out morality.

Study Buddhism and you will find a fascinating world of psychology, sociology, humor and a great amount of intelligence. The Buddhist model doesn't include a creator, which is the most significant difference to other models of religion. Have you ever listened to Alan Watts? I deeply recommend his lectures, they are fascinating and give new and extraordinary insights in the way of thinking.

There are sources which we can take our ethics from. I would go so far to say that without strong ethics within your every day actions - for example programming a computer application, examining a patient, teaching kids, going to the groceries - no matter what you think, do and touch - nothing really is of importance. Or I should say: Ones actions and reactions are in general just random and mostly influenced by other things but ethics. For instance convenience, distraction, fear, boredom, excitement, dullness, .... you name it.

Morality isn't that difficult to find. Mostly within interaction with living beings not so much by theory.
Bummer that I cannot give you my writings on this, because I am a German speaking person.

Good day for you.