So? Is there a point to this part of the question?
It does not say that Seth was their third child. All it really says is that Seth was the first one born after the murder.
Well, Genesis says that Adam and Eve had relations, and Cain was born then Abel, and in the same chapter it goes on to say that Adam and Eve
again had relations and produced Seth.
It's doing a good job of tracking who Adam and Eve conceived. Not to mention the long list of Cain's children. So I see no reason to leave out a verses or anything that suggests that they had more offspring.
And there is not a single verse in Genesis that gives any support for the notion that Cain married his sister.
Cain is afraid of being killed because he is a wanderer, a nomad, and he believes
anyone may kill him as a result. This only makes sense if he is a stranger to them. Why would they want to kill him if they are his brothers? And why would he and God refer to his brothers as "anyone", and not "brothers"?
False - the text does not say Cain met his wife in Nod.
Well it doesn't seem like they had any marriage traditions back then either. The text does seem to say that Cain had relations with a woman in Nod, and the text says Cain left for Nod, not he and his wife, or anyone else left with him.
With the exception of the one that's just false, all your little bits of "evidence" are all arguments from silence.
Don't be foolish and call these arguments from silence, they are arguments from what the plain reading of the texts suggests. Saying that he marries his sister is much more an argument for silence, than anything I have said.