@Jay313 thanks for posting your questions about these estimates. Your confusion here is probably very common, but your effort to closely read the paper helps everyone.
Except there are now. There are now. My estimates are pear reviewed.
They do not explicitly model these changes. Moreover, the discuss exactly what it is that incerases wait times:
- Unrealistically low amounts of migration.
- Very remote islands (which are only colonized within (about) 3,000 years.
- Population size. There is 8 billion alive now, but 5 kya there are just 18 million. 10kya there are just 2 million.
Yes, but that will most likely overestimate the time to UGA. Several types of migration are not accounted for in their model. This one of the big critiques advanced, that they were too conservative.
The variance of the IAP is very high. The variance of the nearly IAP is much much lower. So 15 kya is not what the range for the nearly IAP is.
This calculation is not valid on the high end of the range, because the variance of nearly IAP is much lower than the variance of IAP. Moreover, for a couple in Mesopotamia, the IAP is not really relevant. Only the nearly IAP is relevant, because that is a nearly ideal position. Taking into account the population boom of the agricultural revolution that spreads across the globe from here about 10 kya to 8 kya ago, we just expect that Adam was ancestor of everyone when civilization begins. As knowledge of agriculture spreads, so would his ancestors.
Note, the Rhodes paper does not at all consider the population boom of the agricultural revolution at all, which caused a massive amount of global migration. They do not consider any population level migration at all, which would reduce the estimates dramatically.
I did give ranges (see figures). I also erred often towards more conservative estimates. The goal was to give a likely estimate (but also conservative) given what we know now. I also discuss how those estimates could be improved.
This is only applicable if, for example, Easter Island and Hawaii needs to be colonized. For the time ranges we consider in theology, that makes universal ancestry much easier.
I will point out too that this only matters if you must place Adam 6 kya. Most people who want to preserve the genealogies are not trying to do this. They just want Adam within 10 kya and in an agricultural setting. In that context, there is just no difficulty at all. It would be difficult to imagine how they would not be ancestors of all without positing genealogically isolated populations (which are unobservable).