1)
Why did he ask for a musician?
1.
Rashi (from Pesachim 66b): The Shechinah left him because he got angry. Radak - the music would return him to Simchah.
2.
Radak (based on Shabbos 30b): From the day that Eliyahu departed, Elisha did not have Ru'ach Nevu'ah, for he was mourning. Ru'ach ha'Kodesh comes only amidst Simchah.
3.
Malbim (14): Now, there is a Rasha and a Tzadik in front of him; the matter is equal. Something is needed to veer the matter [to enable Ru'ach ha'Kodesh]. If not for Yehoshafat, music would not help. If not for Yehoram, music would not be needed!
