In Zevin's defense RE character development (for Fikry anyway), I felt that despite his self-destructive tendencies Fikry seemed ready for an excuse to change. It was clear how much he had loved his wife and missed her and then went on a long bender of self pity. The fact that she had been pregnant when she was killed added credibility to the notion that he may have been getting mentally prepped to be a father. So I didn't find it much of a leap to accept his ready response to fathering Maya.
As for Maya, I too felt she was more a prop, character-wise. For one thing she appeared to have too little angst over her abandonment. Abandoned kids usually have more behavior issues. She had none. But then it wasn't her story.
How about the other characters? Did you find Ismay or Lamiase credible? (D0nna Chavez)