Looks like past research plus this thread, particularly the Aleppo connection, wraps up most of the first-level answers to Q's questions; like the pyramid, there are more levels of answers to come. I'll add my own thoughts. A keystone can mean a capstone on the top of a pyramid, although Q uses the word in other ways too. A pyramid without a capstone represents unfinished, ongoing labor (see dollar bill, Masonry); with a capstone it represents work completed under whatever god is symbolized by the capstone.

Obviously Egyptian pyramid symbolism followed the same demon-inspired cults and so Epstein was largely modernizing old methods. Pyramids didn't actually have that many chambers, they were mostly stones; but the chambers themselves allowed human "protection" for the idolatry and crimes against humanity committed there. I'll quote again the taunt of the rich in the end time, calling to the mountains and rocks, “Fall on us and hide us from the face of him who is seated on the throne, and from the wrath of the Lamb, for the great day of their wrath has come, and who can stand?” Note they are trusting in their mountains to hide them from what they misconstrue as "wrath of the Lamb" (lambs are not wrathful). For the unspeakableness of the things they do there, see Ezekiel 8-9.

But the outer shape of the "keystone" is a cube with a dome top (both motifs used frequently but not solely in Islam). The dome represents the womb (as contrasted with the obelisk), and is often compared to the seats of power in the Capitol and the Vatican (and sometimes London). The particular color alternation above the door reminds me of the mask of King Tut, which represented rays around his face and which stands for the glory of God (shekhinah) granted to those in whom he is pleased. Satan, the angel of light, does masquerade this glory on occasion. Hope these neglected aspects help.