Seems to be some kind of parallel between 1) an arrangement made with the hitman and then Diane leaving the key on the coffee table, and 2) Coco telling Diane there must have been an arrangement made and then giving Diane the key who then puts it down on the coffee table.
I was wondering where the hitman left the blue key for Diane - I'm sure not in her apartment on the table. Maybe it doesn't matter, or maybe it was left behind Winkies. That would explain the horrible images Diane associates with that spot. Also, at the diner, hitman pulls the blue key out of his left shirt pocket, same place he told the girl to get the cigarette in the parking lot.