1.) Interesting about sedentary hunter-gatherers during the upper paleolithic -- I'd like to know more about that! But my argument would be that even if the the people were settled, the climate wasn't -- and that the severe climatic fluctuations which were happening would have been enough to deter the incremental steps towards agriculture.
2.) I presume Africa would have experienced the same climatic fluctuations as everywhere else. This begs the question of why they didn't develop agriculture 120k BPE, during the last interglacial? I'd argue it's because we still had some evolution to do to become behaviorally modern, around 80k BPE. So this interglacial is really our first time at-bat with a stable climate.
3.) True, but if they passed through the same bioregion/climate at the same time every year, then that's enough of a fixed point to begin experimenting with agriculture. My theory is as much about the mobility of the bioregions/climate as the people.
4.) Could be a counter-argument indeed. I know too little about New Guinea, which is a crazy fascinating place. I know that there was very early cultivation there, which was also abandoned some thousands of years ago. Don't know how that relates to its history of climate stability. Do say more?
1) We know sedentary hunter-gatherers existed throughout the upper paleolithic. This is a bit of an under-researched area for various reasons, but we know they definitely weren't all mobile.
2) Humans had been continuously inhabiting areas in east Africa since even before they could be called anatomically modern. Yet, agriculture there emerged at roughly the same time as elsewhere.
3) The earliest agriculturists were semi-sedentary anyway, not predominantly sedentary and fixed in place. How mobile they were depends on where and when we're specifically talking about, as it varied over time.
4) Climatic variability in New Guinea (another early center of agriculture) actually increased during/after the younger dryas.
This one's a bit unfair, as it's outside the scope of what I originally said. Nonetheless, it'd be nice if we could also use our explanation to say things about independent domestications long after the start of the holocene, like the eastern agricultural complex.