1)
Who greeted Yehu?
1.
Radak: It was the greatest of the officers.
2)
Why did he call him a Meshuga (lunatic)?
1.
Radak: They would call a Navi Meshuga, for at the time of Nevu'ah he acts like a lunatic - "also he removed his clothes" (Shmuel I, 19:24). He loses his senses and powers of his soul. Evil Yisraelim, who followed Ba'al, disgracefully called Nevi'ei Hashem 'Meshuga'im'.
3)
What is the meaning of "Atem Yedatem Es Sicho"?
1.
Rashi: Do you know that he is a lunatic?
2.
Radak: Do you know him and his words (message)? He did not tell me anything!
3.
Ralbag: You understand that there is no content to his words, so you should not ask about them.