When Elizabeth is watching the video on how to use a handgun, the instructor verbally says and physically loads two rounds into his magazine. He then proceeds to fire one round and the slide locks back as if the magazine were empty.
It is not possible for the hacker to erase Elizabeth's hard drive in just a few seconds, especially by software means. It would take several hours to make the data completely unrecoverable.
When we see the shot where we see the view from Elizabeth's computer and the hidden camera in her vent simultaneously, she raises her arms which blocks the vent from the web-cam's view. However, the laptop is still in full view from the hidden camera's point of view. It is revealed earlier in the movie that the laptop is using a built in web-cam. This means that if the web-cam cannot see the vent, the camera in the vent should not be able to see the laptop either.
The movie said it takes place in Valencia. But when the cops are coming out of Elizabeth's sisters house after the killers leave, the cops car have the LAPD logo but Valencia have their own police not the LAPD.