When did the Hefshet (the skinning) take place? Why does the Pasuk not mention it?
Ramban, Moshav Zekenim: The Nitu'ach (the cutting into pieces) always takes place after the Hefshet, and the reason the Torah does not mention it here is because it mentioned it above in the Parshah of Olah (1:6).
Why does the Pasuk mention the burning of the head, the pieces and the fat before the washing of the innards and the legs, even though they were all burned at the same time?
Ramban, Moshav Zekenim: To teach us that the head, the pieces and the fat did not need to be washed.
Why didn't Pharaoh's Chachamim to try to replicate Arov?
Riva (7:11): Pharaoh did not call them, for he already saw from lice that the magicians could not do so.
Moshav Zekenim (12): The magicians saw from lice that it is due to Hashem, and their actions will not help; it is a waste of effort.