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 Par'oh?
Seforno: To make it clear to him that the plague was from Hashem, and not a natural phenomenon, as a plague of boils generally is.
Why, by the plague of Sh'chin, was Moshe told to throw soot from a furnace
Oznayim la'Torah: Because the four fistfulls of soot from the furnace reminded Hashem of the Mesiras Nafesh of the four Tzadikim who sanctified Hashem's Name 1 by being cast into a furnace
Why did Hashem instruct Moshe to throw the soot "in front of Par'oh", which he did not do by Kinim and Choshech?
Seforno: So that Par'oh should know that it was sent by Hashem and was not a natural plague caused by the elements
Why does the Torah say "K'chu 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.
Because it was the only Makah which required using another item to bring on the plague.