You asked :
"what when and who..begets first entry into completed GP?"
The mainstream historical record says Ma'mun.
Ma'mun, although the Caliph of Baghdad and presumably quite rich, wanted still more treasure. For some reason he thought that the Great Pyramid was full of pharaonic treasure and so decided to break in and steal whatever he could.
Big structure. Where to start?
Ma'mun's a moslem. And from a faraway land. Not going to get too much co operation from the Pyramid gatekeeper. But, lo and behold, Ma'mun's lackies begin their dig just 33 feet below the true entrance. What luck!
Now either, backsheesh accounted for the principal gatekeeper, or, the secondary entry point was already known and less backsheesh was required to pay off some underling. Nevertheless, Ma'mun gets the credit for being the first to enter the pyramid since its construction.
But, what about the true entrance? And the fabled swinging door? Why have a swinging door if not to gain entry without all that digging? And what happened to that door?
So, it appears that someone knew about that door and this knowledge was handed down for centuries. To the ongoing Gatekeeper who may have been of the old religion.
So, one can enter the pyramid easily but cannot access the upper chambers.
Then, what is the point of the door? Just to access the Subterranean Chamber? That portal is a monumental construction feat just to allow someone to crawl down that small shaft. It's gigantic!
Maybe Khufu had a big coffin.
His pallbearers would have had to have shoved him up through the narrow entrance to the Ascending Passage which is now blocked by the Granite Plugs.
Makes a lot of sense when they overengineered the Grand Gallery for the same purpose, doesn't it.
Methinks some revision may be in order.