“1. Would the alleged conspiracy have required a very large number of people to cooperate to pull it off?
2. How likely is it that they could all have kept silent indefinitely?”
In the world we are accustomed to true. But maybe it isn’t always as we envision it. What if there are confederations of very dedicated leftist who believe that they are doing the world a favor by helping to get rid of guns. What if they get a thrill out of fooling us. What if they have a pact that he who defects dies. Who knows they may even be devout Satan worshipers. Remember the bigger the lie the more people are likely to believe it.
You'd basically have to believe that everyone in Newtown, Connecticut belonged to this cult. Along with all their friends and family members. Along with all the teachers and and lots of law enforcement personnel and officials from adjacent towns, the state, the legal system, the FBI, etc.
You'd have to believe that all the members of the (at least) 20 churches in Newtown, Ct. belong to this Satanic cult, for which there is zero evidence.
You'd have to believe that the world is like the Truman Show.
Most people would see this alternate way to "envision" things as absurdly complex and paranoid.
In the absence of other evidence, the simplest explanation makes the most sense. Big lies are not always more believable; sometimes they are more obviously ridiculously false.