"What is the keystone?"
POTUS unlocked the door of all doors (the White House door) using the Key of Military Intelligence. The Stone is the force that can yield its strength to him, their Commander in Chief, using the power of the information provided by the Key.
Thus:
The keystone is the dignity and honor of the office of the President of the United States.
The keystone is the power wielded by the Commander in Chief of the US Armed Forces.
The keystone is embodied in the person of TGE Donald J. Trump. The man with the plan.
Q said this op has been 3 years in the making (thus beginning late 2014). The announcement for candidacy was July 16, 2015, but preliminary financial arrangements/divestitures had already been made (that takes time). Obviously TGE was working with select individuals long before he made his announcement. Given my military experience, I'm fairly sure contingency plans have been brewing for years, constantly being updated, waiting for the right man to come along.
Whether he was recruited or not, TGE chose to embark on that path and with the help of the key, opened the door of all doors. Now he is engaged in a silent war with those who would enslave us and do us harm.
Thus, the importance of the video:
https:// www.youtube.com/watch?v=G2qIXXafxCQ
That speech reveals the mission that was fully formed at that time and he believed it could succeed. TGE is a master strategist, but the majority of the logistical and operational planning had to be done by MI as a military operation, using all assets of the military, but such an OP requires a Keystone, the one piece that holds the arch together and allows it to bear up under much pressure.
MAP = Military Action Plan
MAP
Think MEMO.
The MEMO is an example of a solution to the "classified intelligence" box that holds critical information, but it's a violation of law to reveal such information to the public. Meanwhile those who wish to keep the public under the spell of the narrative can lie all they want, knowing the truth can't be told without the "leaker" being subject to prosecution. The MEMO is an attempt to break out of the "classified" box and get the truth out. Spreading crumbs to people who enjoy digging for the truth and enjoy it, an anonymous hive-mind of equalists who refuse to recognize a leader. Everyone is a leader. None is a leader.
In other words, a perfect group to do battle for the hearts and minds of a nation that's been lied to all their lives, kept blind to the truth by the narrative that's been fed to them as they've become more enslaved and impoverished every year.