How to fix the yandere flag: don't let her talk and tell her he is just indulging the kid with her passing curiosity of having a relationship with an older guy, and that at some point she will feel bored of it and will break it up by finding a younger boy. Say that and maybe things wont be as bad and she will stay quiet about the relationship. Now about the Blondie, hopefully he doesn't know who MC is and just thinks is some random ajussi cuz then he won't directly go attack him and MC might just find a way to get away from him.