Which of the two fatwas should I act upon regarding making up for the Ramadan fast due to masturbation while fasting, given the doubt about knowing it was a fast-breaker?

There is no contradiction between the two fatwas. Whoever was ignorant of the prohibition of masturbation and that it invalidates the fast does not have to make up the fast. However, whoever knew of its prohibition but was ignorant that it invalidates the fast is not excused and must make up the fast. The questioner's doubt about knowing that it invalidates the fast has no effect as long as he knew of its prohibition.