A very interesting post, and you raise some- to my mind- quite pertinent points.
I would like to add a consideration that delves into the common impulse that might propel both diffusionism and hyperdiffusion.
If, humanity developed from a singularity- an archetypal Eve somewhere in Africa- and diffused to every continent in a time span of [+ -]150,000 yrs which seems to be the currently accepted theory by 'those in authority';
what exactly was the impulse that impelled these proto-humans to keep seeking new horizons, to keep moving into ever greener pastures even when they might have been entirely inhospitable- the deserts of central australia.
I find it naive in the extreme to suggest they were merely governed by appetite- abundence or otherwise of food; overpopulation of a particular area. Almost all other species would appear to adapt and stay put in any area that provided nourishment; and within the historical human species, the tendency of the great majority of homo-saps is to stay put in whatever region, country, nationality they are born into.
So I ask again, what impulse impelled these early humans to cover the earth in that remarkably short timespan.
And my second related query is:
What even stranger impulse compelled those now continentally separated races evolved out of that supposed common origin somewhere in Africa, to suddenly begin building impossibly gargantuan structures in stone all over the earth in a roughly similar timeframe?
ps: [It is not my intention to divert interest from your well-made points Scimitar, but I feel these questions are fundamentally related ].