Yes. And to add to, in the flashbacks during the scene when Mr.Grainley was with his dying wife, he gifted her an antique bell and then he says that whenever she wanted him to come to her, she just had to ring the bell. So when he heard the bell ring and then woke up and found the bell outside the basement he felt that his wife wanted him with her. So that along with the other hauntings like the door that keeps shutting on its own, the song that kept playing etc, made him believe that it was all a sign of her presence and that she wanted him with her. Finally the bell made him believe what he felt throughout.
Yea I think the movie was very clear about this, so I'm not sure how some people missed the many cut scenes.
Each time the kids did one of their haunting tricks (door, music, glass, bell), it was followed by a flashback which showed what the old guy was thinking and why he didn't freak out. It was a coincidence, but each one reminded him of a very special moment in his life.
1) Broken door: when his wife was still healthy and complained he didn't fix it. He busted the door with an axe because he realized he should have treated his wife better while she was alive
2) Music: dancing at night. This must have been right after she was diagnosed with cancer and he started realizing that he shouldn't be angry with her and cherish the time they have left.
3) Broken glass lets the cold in: the day she comes back from the hospital and they prepare the room for her. It was cold that day because it was winter.
4) Bell: he's fully devoted to making her last moments happy and as arvnd9 says, he told her he would come anytime she rang the bell.
I was thinking the cat did it. Obviously he was eccentric enough to will the entire estate to the cat due to his sentimentality. The cat, sitting there and seeing both sides of the experiment knew he just needed to knock a camera off to trigger the whole end scene. Cats are freakin' smart AND devious that way.
So yeah, the cat did it. They hate humans and just want our inheritance