A couple thoughts:
There in nothing in Genesis 4 - 6 that indicates that Abel had any children. The Bible records the descendants of Cain; so it makes sense that it would also document the descendants of Abel if there were any. Nor is there anything to indicate that Adam and Eve had any children between the death of Abel and the birth of Seth. when Adam was 130 years old. In fact, Gen. 5:4 says that after Seth was born Adam and Eve had other sons and daughters.
Also, the scripture verses you gave all came from a time period after Cain had killed Abel.
Those "later building sites" were probably cities built on top of the ruins of previous cities.
You used some "maybe"s and "probably"s in your post. Without Biblical evidence, maybe and probably is as close as either of us can get. Preflood Biblical history is pretty sketchy. Even though the stories in non-Biblical pre-flood mythology is corrupted with a pantheon of diverse gods and demigods ( the Bible describes them as fallen angels and Nephilim) rather than the one true God, they nevertheless document a pre-flood, flood, and post-flood history of scores of ancient civilizations all over the world. I think we would be wise to at least look at some of these "myths" - especially those involving the ancient civilizations of the Middle East and Europe (e.g., Sumerian, Egyptian, Greek, Roman) to gain a better understanding of pre-flood history.