1)
Why does it say "u'Va', and not "va'Yavo"?
1.
Malbim (5): This shows that he was prepared to go and come to there. He came opposite [David], and once he left the city he began to curse. 1
1
Malbim (6): However, he began throwing rocks only when he came close to David.
2)
What is "Bachurim"?
1.
Radak: It is a city in Binyamin. In Divrei ha'Yamim (1:6:45) it is called Almas, like Targum Yonasan here. These two words have the same meaning (youth).