With or without gun control the probability of a person right there just happening to have a gun would be extremely low. And then the probability of that individual being able to do anything about zero due to the rapid nature of the crime. You put the two together and the probability of someone being able to do anything about this crime is virtually zero. So a discussion of gun control is irrelevant to this specific incident.
Not really. VA is a very gun friendly state. The fact that a student was taken to court by the university for carrying a gun, with a legal CCP, shows that there were students who did.
The bottom line is that permitting gun carry gives a possibility of defense prohibiting it, as was the case, changes that to no possibility. Your speculations on the odds are irrelevant to those facts.