Salam aleikum There where many questions in your post. Maybe we can discuss them one by one. I think, and Allah knows best, that the analogy is based on the following: 1 A living person is doing an action that benefits the dead 2 This action involves the establishment of dhikr If dhikr made by a plant benefits the dead, then the reading of the Quran (the best dhikr) will also benefit the dead.