What is the meaning of "Im ha'Melech"?
Rashi: With Avshalom. Radak - he is called king, for Yisrael wanted to make him king.
What is "Nochri Atah", and why is it a reason to return?
Rashi: You are a Goy. We are fleeing without provisions. You are a Goy, so people will not have mercy on you.
Radak: You are a foreigner, for you lived in Gas a long time. Perhaps he was born there.
Malbim: You are a Goy. If you go because you fear Avshalom, you can return to Yerushalayim and the king. You are a Goy, so he will not pay attention to you.
What is the meaning of "v'Gam Goleh Atah li'Mkomecha"?
Rashi, based on Targum Yonasan: Also if you want to be exiled from Avshalom, for you do not want to be with him, return to your place, for it is not good to go with me.
Radak: Goleh refers to being moved. Usually Galus refers to captives being moved. Sometimes it refers to movement without captivity, like here, and "va'Yaglum El Manachas" (Divrei ha'Yamim I, 8:6). "Li'Mkomecha" refers to "Shuv v'Shev" - return to your place and dwell with the king in your place, i.e. Yerushalayim. He came from Gas to dwell in Yerushalayim.
Malbim: If Avshalom will exile you, you will return to your place in Gas, and not lose anything.