How do we explain "Haloch Halchu ha'Etzim"?
Rashi: It is a parable. Malbim - trees are a metaphor for people - "ha'Adam Etz ha'Sadeh" (Devarim 20:19).
Radak citing Targum Yonasan: It is incredulous - did the trees go...?! He similarly translated (verse 9) "he'Chadalti" - ha'Eshbok (will I abandon my oil in my place)?!
What does the olive tree represent?
Rashi: It refers to Osni'el ben Kenaz from Shevet Yehudah, which is called Zayis - (Yirmeyah 11:16) "Zayis Ra'anan Yefe Pri So'ar" (Tanchuma Yashan Vayera 29). Bereishis Rabsi (20:15) - also his Torah illuminated the world like olive oil.
Malbim: There are three purposes - good, pleasant, and beneficial. First they picked the Zayis, which produces oil for light, which is a metaphor for Chachmah and Torah, and light is called good (Bereishis 1:4).
Why did they say only "Malchah"? To the other trees, they said "Lechi At Malchi"!