Great post.
Two genius Blacks have children = average IQ will still be 85.
Two White trash have children = average IQ = 100.
^ [over successive generations].
We are the sum of our parents, in a way, but more like: the sum of all our ancestors.
Not suggesting we take in Africans with a mean of 100. I would never let anyone into my country with an IQ less than 100 ever. The lowest score you can have as an immigrant, should be in the 110-115 range. Yes regression to the mean is a thing, but it's not called "return to the mean" for a reason. From what I've seen, like most things it's a mix of both.
If your IQ is just averaged from your parent's IQ, the US military would breed a race of super geniuses. But gene's do matter and I would imagine that over successive generations weak genes get weeded out(assuming your environment forces weeding out the stupid). I could be wrong but the IQ of your grandparents is actually a far better predictor of your iq than your parents.

It's like a hand in poker. You can get a high score by having the right combination of cards(genes), but unlike poker you get to mix your hand with your partners hand.
So if your dad had a straight flush his cards won't give you any real benefit when mixed into your mothers hand. But if your mother is winning by pairs, odds are after time you're gonna have fewer 2s, 3s, 4s, than you have jacks queens kings.