You're really over-thinking it.
The "Keystone" is what holds the bridge together. When you place the keystone, the bridge is secure. When you remove the keystone, the bridge falls apart.
Epstein is the keystone. Once all his information is revealed, the entire deepstate falls apart.
Linguistically, even his name indicates a stone of higher importance: Epi = higher, Stein = stone.
So the double meaning confirms this theory.
Lastly, Q also references the USA being a keystone.
In other words, exposing the deepstate in the USA is the keystone that exposes the cabal world wide.
Epstein is the keystone that unlocks the USA. USA unlocks the world.
Epstein is the keystone that unlocks ALL, and brings down the cabal world wide.