Question: Are there any clues throughout the movie hinting that the old woman is the devil?
Answer: No. The devil is a deceiver and a trickster who can easily manipulate anyone so everybody in the elevator would not even think to suspect an old lady.
Answer: Yes, there are many clues actually. The old woman's reflection throughout the film always had black eyes. In one scene, they're backed against the wall, the elevator buttons look like devil horns on her head. When she goes to push the floor button, the color is red, rather than the green it is for others. The other two answers just don't pay attention.
Question: When Tony is trying to close the dead old woman's eyes, the security guard states the body has to be dead for a few hours in order for the eyes to close. Is this true?
Chosen answer: Yes. When the body dies, gasses begin escaping through any way they can, and can force the eyes to stay open. This is also true for a body that has been dead for several hours, once decay starts, the decaying causes gasses. But also when the body is freshly dead, the muscles and nerves are confused and keep firing for a long time, causing muscle spasms like feet and hands wiggling on the deceased, or even eyes popping open suddenly as if the person is awake. This is why the eye lids are sown shut on bodies at funerals and stuff, to prevent the eyes from popping open and freaking out the people viewing. And in older times, coins would be places over the eyes to hold them shut.
Answer: There weren't any clues. It comes off as a total surprise during the reveal. But watching it for the second time, I did notice that she spoke very less compared to others and no one was suspecting her. Usually, that kind of character ends up being the bad guy.
Earthling