Hahah, The Bible does not have to be true or logical in itself...the Christian-Judaic-Islamic faiths state initial conditions, and base all "logical" thought on this...
1: There is a God
2: There is a book written by our God through people he directly inspired to write them.
3: All MAJOR concepts in this Book are true because God wrote them and God knows and controls all.
That's it, theres nothing more any religious folllower needs to argue with a non-follower....The debate will never end, so whether or not it makes SENSE that the bible leaves out SOME things is irrelevant...It's the major concepts that count.
Whether Cain was the only one left with Adam and Eve or if there was a quickly mentioned place called "Nod" that contradicts the other, more incomplete chapters in the bible written by other people also inspired by God is wholly irrelevant to the fact that you must accept Jesus Christ as your savior and do good works for eternal happiness...we know this based on the 3 statements above...
No one can LOGICALLY deny these three assumptions as true, because they're statements assumed as true. Just as it is ASSUMED that 1=1 2=2 1+2=3 etc. you can't actually PROVE these statements as true or false...they're based off of initial conditions in number theory that cannot be altered...