Legal marriage age for girls in Japan is 16, if all parties including the parents agree to it. Age of consent varies more, and generally not accepted below 18.
No conflict whatsoever. Why is she even distant? Good relation with family, good relation with friends and others, good at everything she does. What is the problem here even? Where is the character growth?
I dont think the author is excusing the mom at all, rather the message is despite whats happened in the past you need to try to move on or else things will just get worse. I doubt the intention is to make the mother come across as a victim.