Apparently Pat had planned on painting that door the very next day, but Laura was dead against it! Hehehe.
Seriously though. When Pam came back, she had trouble opening the door because Laura was upside down against it on the other side. Surely she would have reported how she found Laura like this to the police at least. In that case, how could anyone have killed Laura, gotten out of the house and ensure that the body was still blocking the door from the inside? It wouldn't take Sherlock homes to workout that she had fallen accidentally, especially given the wheeled toys at the top of the stairs (and probably one at the bottom). The evidence could only show that Janine and Laura hated each other (Ian hated her too) but that an accident had happened.
I suppose the police will note that Janine's husband had recently died from an unusual fall, and suggest that she got rid of Laura the same way. However, as Ian has admitted going over there around the time of her death, and she was looking at Bobby's birth certificate (with his name on it), and writing Ian loads of scrunched up letters, they could also easily assume that he went in, was told of his son and shoved her down the stairs. Then of course, left saying there was no answer when he rang.
His story, and the apparent evidence against him, will be the most damning, and his fingerprints will be the last on the doorbell too. The police have checked for this haven't they? As they should in any suspicious death.
Rudi