What would Jonah have answered God at the end of the Book? Did Jonah learn the lessons?

Since we are not told, to speculate would be useless. We can see the lessons God taught Jonah and it is those lessons which we, in turn, also need to learn. As Paul warned, "that you may learn in us not to think beyond what is written" (I Corinthians 4:6). God did not deem it important for us to know whether Jonah himself learned those lessons or not.

See also:

Lessons from Jonah
Questions and Answers regarding the Old Testament