Is it permissible to build a mosque and a water well and to include six deceased grandparents in their reward, and will the reward for that reach them?

Donating the reward of ongoing charity (Sadaqah Jariyah) to the deceased is a good deed, and it is permissible for more than one person to share in this reward. As long as the reward reaches them, it does not matter whether they know it came from you or not.