I like to imagine that Nolan was always a little dissatisfied with the Viltrumite Empire, even if it was subconscious. He would still be completely unwilling to turn traitor until after his fight with Mark, but I imagine he wrote their weaknesses into his books because some small part of him hoped they would eventually be found and read by people capable of resisting Viltrumite rule.

He just started writing because he had to blend in and in order to do it he needed to work. What better thing to do while being thousands of years old than to write your story?

The thing I don’t understand is why he got married to anessa and had a child with her. Is that ever explained? The thought process behind it? Or is it simply looked over?

I think he intended them for Mark to use to understand potential threats later down the line if something where to happen to him. Than once the tides turned he realized Mark could also use them to take down the Viltramites as well.