Why did Adam and Eve's grandchildren not have birth defects from inbreeding?

You cold ask the same about Noah, or about any local population that ever experienced a genetic bottleneck. And the answer is that birth defects do not always result from inbreeding, especially if it is brief. Adam and Eve had many children (Genesis ch.5), which created a diverse population immediately.

This, together with the fact that God gave them robust genetics to start with.

That is an unanswerable question. Most of the time the answer will be "because God wouldn't allow it to happen".

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.