If a keystone can also be non-archititectual, but figurative, like a "keystone species" that is required for a whole ecosystem to survive, then who is your choice for a person who seems to be central to all the other subjects here?
For a person, I would pick GHWB due to connection to JFK, CIA, GWB, 911(via W), was running the show behind Reagan for 8 years, then was President, handed the reins to Bubba(gave Bubba the Presidency for his service as Gov of Arkansas to fund CIA ops out of Mena) on to probably puppeteering W's presidency…
He has the longest ties to the past and the present than any if them.
Can anyone continue the list of his ties going through Obama?