Why did they ask Baruch how he wrote the Megilah?
Malbim: The officers in the days of Yehoyakim already decided that Yirmeyah is not Chayav Misah for his Nevu'ah, for it is from Hashem. They were unsure about Baruch. Perhaps he himself decided to copy these matters from Yirmeyah's Sefer, without a Shelichus from Hashem. If so, he is Chayav Misah 1 !
Presumably, he is Chayav Misah only if when he read it, he falsely said that Hashem commanded him to read it. Did the Sarim think that he would admit this to them?! Perhaps when he read it, he said 'so prophesized Yirmeyah', and he would admit that Hashem did not command him. Since the prophecy included that the king will be exiled, perhaps it is like rebellion to publicly say so. (PF)