It's still environment, even if it's not practically measurable! Also, handedness can modified [by changing the environment].

I don't think that's right. The number of pairwise differences is proportional to the TMRCA. a and b cannot coalesce until before the split (at which time all populations are symmetric) so E[T_ab]=E[T_bc] (=T+2N_anc) and does not depend on what happens on the A lineage.

I think it's more to do with the structure within Africa; if you think about the tree, it's clear that e.g. Non-AFR vs East AFR should have fewer differences than East AFR vs West AFR etc. I don't think the OOA bottleneck changes the expected number of non-AFR vs AFR differences.

Isn't this more to do with sampling than demography, and a consequence of only looking at the first 2 PCs? e.g. compare 1000 Genomes AFR populations on PC1-4 vs 5.

Not much, I guess the only thing I'm aware of is that EDAR V370A is common in East Asia 10-20kya, so presumably straight/thick hair: www.sciencedirect.com/science/arti... (Fig. 4)

Uhh….