Why do French macarons need two types of sugar?
As this answer was originally downvoted for not being helpful answering all 3 questions in that one single question separately:

a. Powdered sugar contains additives to ensure it doesn't re-crystallize into a big hard rock of sugar under normal atmospheric conditions and only using that one would give the macarons a powdery taste.
b. Using granulated sugar only would change the texture of the macaron as all of the sugar cannot be dissolved in the egg withes.
a. Yes: the undissolved powdered sugar gives the macaron its stickly sweet taste while munching
b. the dissolved granulated sugar gives it it's first hit of taste while biting down.
Making macarons is as much Black Magic as it's a science: even when carefully measuring everything and doing things perfectly, they sometimes don't turn out perfectly, so:

No! Please! Don't change anything!