On point #1, Adam and Eve had lots of kids including girls, I think we have settled that they married the sisters. And that it was OK at the time, because it was God's plan that this happen. It was only much later that incest became bad.
"After Seth was born, Adam lived 800 years and had other sons and daughters. Altogether, Adam lived 930 years, and then he died. "
I mean, Eve was made *from* Adam so she was a clone of him, if you want to start from there. And it was OK for them to have sex. That's just how God wanted things.
In fact in Genesis 6 it says:
"When men began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of men were beautiful, and they married any of them they chose."
So it says right there that they were marrying any they wanted
Right after the Bible talks about Cain and Abel being born, and Cain killing Abel, it says:
"And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch."
So Cain already was with his sister at that point.