What is "Nir"?
Rashi: It is kingship. Radak - Yonason translates so, and also Onkelos translates "va'Niram" (Bamidbar 21:30) as 'and its king.'
Radak: This is like Ner with Tzeirei, in the way of "Arachti Ner li'Mshichai 1 " (Tehilim 132:17)
Radak (Divrei ha'Yamim II, 21:7): The king is like a lamp, that illuminates. This is like Ben and (sometimes it is written) Bin, Rash (poor) and Rish (e.g. Mishlei 28:19).
What was Hashem's promise to David?
Malbim: Regarding his sons, "if he will go crooked, I will rebuke him; My Chesed will not cease
So he is called in Divrei ha'Yamim II, 21:17, 25:23, 25. Elsewhere he is called Achazyahu. Perhaps Malbim did not mention Amon ben Menasheh, for Menasheh repented before he died. However, in any case he could have mentioned Yechanyah ben Yehoyakim. (PF)
Malbim explains "Vanav" to be David's seed whose fathers were Tzadikim, but they themselves soured. I do not see how the verse hints to this. (PF)
