I'm more and more convinced that what mieruko is seeing is not just ghosts, but also demons, hence why others can't see them, my real question and probably something that will eventually come up, is why mieruko can see them.
Also i don't think anything is gonna happen to granny, this series has never really delved into tragedy yet and if granny died logically mieruko who would have seen her only hope die with her, in a more realistic term probably wouldn't be able to handle it.
But even then, i honestly don't think the author wants to take that route, more than likely grandma is gonna be a part of the main cast from here on if i was to guess.