1)
Who went to Bavel?
1.
Radak citing Seder Olam 25, b'Sof: In Tzidkiyah's fourth year, he went to greet Melech Bavel. He returned to his kingship in Yerushalayim.
2.
Radak citing Targum Yonasan: Tzidkiyah did not go; he sent Serayah ben Neri. "Es Tzidkiyah" means for the sake of Tzidkiyah.
2)
Why does it say "bi'Shnas ha'Revi'is"?
1.
Radak: The matter counted is normally Samuch to the number.
3)
What is "Sar Menuchah"?
1.
Rashi, Targum Yonasan: The head of offerings. Via him, those who bring gifts see the king.
2.
Radak: The friend of the king. He was with the king at the time of his serenity, to talk with him and enjoy with him.