Why does it say "va'Ya'an"?
Malbim: (a) This was like a response. The tearing of the Mizbe'ach and the drying of my arm do not prove [that your Nevu'ah is true] until you show that you can reverse a matter, i.e. to heal my hand. (b) He wanted to see if Teshuvah can fix matters. If the Navi's Tefilah will help to heal his body's illness, it can help to fix his soul's illness, and he will repent.
Why did he say "Elokecha"?
Rashi (from Tanchuma Ki Sisa 6): He is not my G-d. Yaravam persisted in his rebellion. Radak - how could he call him Elokai 1 , while he is offering to idolatry?!
Radak: You are Hashem's Shali'ach; I know that He will heed you.
Seemingly, we expound because he did not say Hashem Elokim, or say only 'Hashem'! (PF)
Why did the Navi pray for him?
Radak: He was afraid of him.
What is the meaning of "va'T'hi keva'Rishonah"?
Rashi and Radak (from Tanchuma Ki Sisa 6): He stood and offered to idolatry. If you crush a fool in a mortar, it does not help at all!
Radak: He was able to move it as he wanted.
