Would'nt an alien race and/or advanced civilization have known how to build the ceiling of the kings chamber? As I understand it there were apparently flaws in the initial design, a broken 'roof stone' and an ugly kludge for reinforcement.
As far as I have seen the broken lintel occurred after construction perhaps due to earth quake. I never speculated on who built it or why.
While you wonder about the broken part I marvel that the King's Chamber a)never had a body in it; and b)was walled in marble. Paul Horn's flute recording from inside the Kings Chamber are really interesting.