The CSI tech at the murder scene shows the police a pristine bullet that went through a door lens and presumably a human head without any compaction or fragmentation and without any blood on it, which is impossible.
There is no reason why the video feed in Yao's apartment building was not replaced by a simple one devoid of people or even just a black screen. To go through the process of removing people from a video is difficult and time-consuming, plus (in this case anyway), it leaves the opening and closing doors visible, leading to far more questions than the first two options.