Is seeking forgiveness for the deceased 100 times daily with the wording: "O Allah, forgive him and have mercy on him" sound, even without believing in the virtue of the number or the specific wording?

The Sharia encourages praying for one's parents, which is a great good, as stated in His Almighty's saying: "And say, 'My Lord, have mercy upon them as they nurtured me when I was small.'" The method used in this supplication is correct, and there is no objection to it.