So it turns out that fusing the Healing Tome with a monster, and then equipping the Unraveling Prism, both removes my monsters' poison and heals all my monsters. 
So it turns out that fusing the Healing Tome with a monster, and then equipping the Unraveling Prism, both removes my monsters' poison and heals all my monsters.
That makes sense, I was wondering why it was healing different amounts when I didn't raise Parra's Int- it was because the amount of healing from the Unraveling Prism wasn't tied to Parra's Int at all! :O