1)
Why does it say "Lachen"?
1.
Malbim: Their punishment will be double, for (a) Prophesizing Sheker in the name of Hashem. Even if they did not make people sin, a Navi Sheker is executed via choking. (b) Their words make people stray from Hashem's ways. The Navi first tells their punishment for false Nevu'ah, for this impinges on Hashem's honor.
2)
What is the meaning of "Hineni Aleichem"?
1.
Radak: This is like "v'Nasati Es Panai ba'Nefesh ha'Hu" (Vayikra 20:6).
2.
Radak citing Targum Yonasan: I will send My anger against you.
