1)
According to the opinion that Palti never had Bi'ah with her, why was he crying when she was taken away?
1.
Rashi (from Sanhedrin 19b): He cried over the loss of his Mitzvah [Radak - of overcoming his Yetzer ha'Ra]. The years that she was with him, he inserted a sword between them in the bed, and never stumbled with her.
2)
Why does it say "Ad Bachurim"?
1.
Radak (from Sanhedrin 19b): He was like bachelors who never tasted that sin (of Bi'ah).
2.
Radak: Bachurim is the name of a place in Binyamin - "Shim'i ben Gera
1
This is based on Radak in Divrei ha'Yamim I, 7:8. Our text of Radak here "Almus ben Ya'areh" is a printing mistake.