1)
Did they hear Shlomo's Chachmah from the other kings?
1.
Radak, Malbim #1: Yes. Other kings came to Shlomo to hear his Chachmah; the nations went to their kings, when they returned, to hear Shlomo's Chachmah from them.
2.
Malbim citing Mahari: No. The other kings who heard about Shlomo's Chachmah, they sent their Chachamim to hear his Chachmah.
3.
Malbim #2: Our verse hints that Sefer Koheles was said at Hakhel. (Koheles Rabah 1:2 - Shlomo is called Koheles, for his words were said at Hakhel - "Az Yakhel Shlomo" - 8:1.) There he spoke in the way of investigation about all matters of the world. Also kings learned from this, that success in this world is nothing, and also the masses of nations [learned].
