Ninth calls Fourth to warn him about his son's heart disease.
Gasai would fall for anyone who allowed her to cling to them. Amano would fall for anyone who protected him.
Clearly the right way to solve this is to seal him in an illusionary world where she doesn't exist.
Causality ripples in a way that helps the Future Diary users.
Twelfth's vigilantism let's him catch Third.
Sixth's parents aren't killed and her cult doesn't go evil. Fifth's parents don't join.
Eleventh sees the end of the game in his prototype diary and tells Deus.