Is it permissible to dedicate the reward of ongoing charity (Sadaqa Jariyah) to more than one deceased person, and does that diminish the reward of the original deceased person for whom the charity was intended?

The correct view is that the reward for righteous deeds dedicated to the deceased reaches them, whether it is an ongoing charity (sadaqah jariyah) or not. It is permissible to include more than one deceased person in the reward of charity. Every act of devotion performed by a Muslim, and its reward or part of it is dedicated to a living or deceased Muslim, is permissible and beneficial to them. Therefore, if you include others with your father in dedicating the reward of the charity, his share will reach him according to your intention, for deeds are by intentions.