The youth potion resets the semen volume back to 0, and it also says it would decrease Monica's overall status but I didn't see any changes made to Attack/Defense/Intelligence/Resist/Agility/Luck.
sinker093 posted a walkthrough on page 39 of this thread. But I can't repost the link since I'm a new user.
As for the balls, I think it's called Powders at the store.
Sleeping Ball -- Sleeping Powder (Sleep 80%)
Bilibili Ball -- Electrified Powder (Paralysis 80%)
Hikari ball -- Glowball (Darkness 80%)