1)
What is the difference between "Megilah" and "Sefer"?
1.
Radak: The parchment on which one writes is called Megilah or Gilyon. Megilah is Samuch to Sefer - this means, write on it, and then it will be called a Sefer.
2)
Why does it say "v'Chasavta Eleha*?
1.
Radak: This is like Aleha.
3)
Was he commanded to write all the Nevu'os in this Sefer?
1.
Radak: Yes.
2.
Radak citing Mo'ed Katan 26a: No. He was commanded to write Megilas Eichah.