Why did he say "va'Yosef" with the prefix Vov?
Radak: The Vov begins a matter; it does not come to add. This occurs often, like I explained in Yehoshua (1:1).
Why does it say "El ha'Am"?
Radak citing Targum Yonasan: This is like Al ha'Am.
Why did he say "Kahem v'Chahem"?
Rashi (from Pesikta Rabsi 11): They should increase like they are now (i.e. double), and then [double] again, [and double] 100 times 1 . This was far greater than Moshe's Brachah to increase 1.000 times (Devarim 1:11). Also, Moshe's Brachah was in the long term, and Yo'av's was immediate - "v'Einei Adoni ha'Melech Ro'os."
I.e. 2 to the power of 100, about 10 to the power of 30. This is utterly beyond nature for so many people to fit on planet earth! (PF)
Why did Yo'av bless Yisrael?
Malbim: This was one of two 1 reasons not to count them; counting inhibits Brachah and increase, either because it is counted, or due to Ayin ha'Ra.
Why did he ask why David wants to do so?
Malbim: This is a second 1 reason not to count them; there is no need. In Divrei ha'Yamim I, 21:3 it explains more - "they are all slaves to [you]." I.e. you think that they will not heed you to come to fight when you call, so you need to count them (refer to 24:13:6:1). You err. They are slaves, and you are their king! If so, there is no need to count them.
