Oda actually confirmed that the five Elders do not have Devil Fruits so it stands to reason that Imu, the source of their power does not have one either
No idea. Maybe the source of Imu's power is that third and final ancient weapon that has not been named or even given any specifics about what it does. Maybe it's something like Pandora's Box that bestowed Unholy power and Imu can pass it around as well