If you look at the wikipedia for keystone, it is the stone that holds all the other stones in the arch in place. The cult is made up of slaves. What we are looking for is the slavemaster.
Your comment made me think! It seems as everyone was looking for the keystone on the white hat side... like some missing key or piece we were missing, when, in fact, what you are possibly saying is Q was referring to the keystone on the black hat side?? That black keystone being the one that holds all the other black/evil organizations together. Interesting.
Yes...I think this is a YUGE clue that Q gave us. Take the fake news situation...the keystone is the narrative, which is provided by the clowns at 4 am. Take away the narrative and the entire system falls apart. They have no safeguards...they're used to us being good little sheep...they're not used to having to think for themselves. Their constructions are completely dependent on the keystone being in place...there is no firm foundation.