There is nothing unusual in this use of calling different group of people the sons of God or son of God. It is a habit in common usage today. The same as terms like brothers or sisters. The full meaning is in the context. God is always ready to accept people and ascribe to them family terms – even Deedat could have become a child of God (too late now).

For part (c) the footnote 3 asks the question how can God beget David at the age of 40 “this day”. Simple. It is a future prophesy and the person it is referring to is Jesus.