During her interview, Natasha Piper's gray top covers both her shoulders, then her left shoulder is revealed from another camera angle. The gray top moves on her shoulder without moving her arm.
During Natasha Piper's interview, her right arm changes from the forearm being positioned on the armrest in shots from the front to her elbow being propped upon the backrest.
At the conclusion they say there are two murderers, however the second person who bludgeoned the dead body did not commit murder since the victim was already dead. You can't murder a dead person. At most the second person might be guilty of some minor crime like desecration of a dead body, or whatever crime the New York Penal code has for that type of crime.
Castle and the detectives assume that since the alarm clock fell off the night stand and broke at a specific time (10:34) that was when the attack started, but it could have fallen and broken at any point during the attack, not necessarily the beginning.
Beckett asks Castle, "What are you still doing here?" Castle replies, "Hey. Just about to leave myself." However, he's in the act of pouring himself a cup of coffee into a china cup. Clearly he isn't just about to leave but Beckett doesn't pick up on it.