Is it possible for a repentant young man who used to pray intermittently, and practiced masturbation during Ramadan, to make up the missed prayers and days since the age of 15? And is it permissible to adopt the fatwa of other madhhabs regarding making up the fasting days that were invalidated due to masturbation, with or without expiation?

You are only required to make up for the days you missed fasting, and no expiation (kaffarah (definition)) is obligatory upon you. The view of the majority of scholars is that expiation is only obligatory for breaking the fast through sexual intercourse. If you are unsure about the number of days or prayers, then make an effort to determine them and make up what you are certain of or what you predominantly believe will absolve you. You are not required to make up for anything from the time of puberty, but rather only to make up for the fasts you broke and the prayers you missed.