Hello Anthony,

You write, 'Either way, we are in very close agreement about what happened here. We're off by a single block of stone being popped out. I say before. You suggest after.

The consequences of the difference are immaterial, really. Agreed?'

Well, I've more or less convinced myself that Al Mamun discovered the start of the Ascending Passage before he started his tunnel.
I think he realised that the two floor joints signalled something different about that particular part of the Descending Passage and set about probing the surrounding floor, walls, and ceiling.
He discovered the block in the roof was somehow different to its companions (a touch of vagueness here) and had it broken out (which explains the block on the floor being a fragment).
Seeing the granite plug and realising that tunnelling upward by the side of it was tantamount to suicide, he opted for a tunnel coming in from the side.

What I'm still unsure about is why he elected to start his tunnel where he did.

Where you and I continue to disagree is over whether the Ascending Passage plugs and their covering block were put in place before or after Khufu's death.
I am certain that the plugs and covering block were built in place, and therefore Khufu's remains and his funerary paraphenalia were not taken into the upper chambers of his pyramid via the Ascending Passage (or the Well Shaft).

MJ