Over which Mes should they not cry?
Rashi: Over Yehoyakim, who will die in front of the gate when they drag him to exile.
Mo'ed Katan 27b, partially cited in Radak: Do not cry over a Mes more than the proper amount - three days for crying, seven days for eulogies and 30 days for [his close relatives to refrain from] laundering and cutting hair. From here and onwards, Hashem says 'you should not be more merciful than Me 1 .'
Ba'al ha'Turim: There is [one] other verse that says "l'Mes 2 " - "v'Lo Nasati Mimenu l'Mes" (Devarim 26:14). This teaches that one may not use Ma'aser Sheni to hire people to cry and arouse wailing over a Mes.
Lev Eliyahu (Shemos p.108): This shows that Aveilus is mercy on the Mes.
He means with a Sheva under the Lamed. If not, there is also "v'Lo Sasimu Karachah Bein Eineichem la'Mes" (Devarim 14:1). (PF)
For whom should they cry?
Rashi: For Yehoyachin and Tzidkiyah.
Radak: For Yehoyachin.
Malbim: For Shalum, the fourth son of Yoshiyah (Divrei ha'Yamim I, 3:15). Verses do not say what happened to him. It seems that when Yehoyakim was killed, he was exiled to Bavel, lest he contest Tzidkiyah's kingship.
What is the grammatical form of "Vachu"?
Radak: This is Makor (like an infinitive), just there is a Vov in place of the Hei (the last letter of the root).
Is it not more proper to cry over one who will die, than over those who will live?
Rashi: This teaches that Yehoyakim's death, about which it says "his dead body will be cast [on the ground] in the heat of day and the cold of night" (36:30) is better than the life of Yehoyachin, about whom it says "va'Aruchaso Aruchas Tamid" (Melachim II, 25:30)! Radak - it is better to die than to live a life of pain and disgrace.
