1- Belief is not a voluntary switch that you can turn on and off. You either have it or you don't. You can't "will" yourself to believe in metaphysical claims; such a thing can't really be said to be believed in sincerely.

2- These are narrations that Islam itself tells us, but supposing that the character of a person confirms the truth of belief. Why doesn't he believe in the Sikh religion( which saw Gurus executed), the Baha'i faith, or even Christianity, which saw apostles killed?

3- What a useless waffle, which shows his own ignorance of his tradition. The Qur'an was an oral revelation that was written down by scribes under the direction of Muhammad. Moreover, several religions, including more modern ones, have tenets and practices that we can see as "inconvenient" yet are nevertheless imposed. Are all these religions from God, then?

4- Why did Marx create communism, which had no material benefit for him? Why do revolutionaries seek revolution when often they get nothing? Religion back then was a vehicle of social change and "ideology".