How is it possible for Moshe to throw his and Aharon's double hands-full of soot (up into the sky [See Sifsei Chachamim]) with force?
Why was it necessary for Moshe to throw the soot before the eyes of Pharaoh?
Seforno: To make it clear to him that the plague was from Hashem, and not a natural phenomenon, as a plague of boils often is.
Why does it say "Kechu Lachem" only regarding boils?
Moshav Zekenim: The other Makos were in the world, just they needed to be brought to Egypt. Boils did not come from elsewhere. One who was decreed to have boils, Hashem would not take them away to bring them to Egypt.