"As a Christian there is no satisfactory answers to be found."
Of course there are! This is an erroneous ASSUMPTION BY YOU, one made by unbelief not belief. I have done the same thing!
Jesus said many times to people like the disciples, religious leaders & people like you & me: 'Have you not read, does the Scripture not say, Do you not know, is it not written, you know neither the Scriptures nor the power of God, with men this is impossible but not with God,' etc.
Holy Scripture is the very God-breathed out words of God.
I Sam 15:29 The Glory of Israel will not lie...
Titus 1:1-3 Paul, a servant of God & an apostle of Jesus Christ, for the sake of the FAITH of God’s elect AND THEIR KNOWLEDGE OF THE TRUTH, which accords with godliness, in hope of eternal life, which God, WHO NEVER LIES, promised BEFORE the ages began & AT THE PROPER TIME manifested IN HIS WORD through the preaching with which I have been entrusted by the command of God our Savior...
John 17:14,17 For I have given them the words You gave Me & they have received them. They knew WITH CERTAINTY that I came from You & they believed that You sent Me. Sanctify them in the truth; Your word is truth.
2 Timothy 3:14-17 But as for you, continue in the things you have learned & FIRMLY BELIEVED, since you know from whom you have learned them. From infancy you have known the Holy Scriptures, which are ABLE TO MAKE YOU WISE for salvation through FAITH IN CHRIST JESUS. All Scripture is INSPIRED (God-breathed out) by God & is profitable for instruction, for reproof, for correction & for training in righteousness, so that the man of God may be complete, fully equipped for every good work.
Those Scriptures above being said, we can with full confidence of faith BELIEVE WHAT GOD SAID in Genesis concerning Cain, just by itself--although it is not isolated from the rest of the testimony of the whole of Scripture, both OT & NT. Don't limit yourself to one passage without the corroboration of the rest of Scripture.
What did God record as His Word, which is THE TRUTH, concerning what happened to Cain? You might also consider things perhaps you didn't notice in the text itself or consider concerning the PASSAGE OF TIME.
Genesis 4:1-16
And Adam had relations with his wife Eve & she conceived & gave birth to Cain. “With the help of the LORD I have brought forth a man,” she said.
2 Later (again) she gave birth, to Cain’s brother Abel.
(It does not say SPECIFICALLY how much time had passed between their two births, only that later IN TIME, was Abel born.)
Now Abel was a keeper of sheep, while Cain was a tiller of the soil.
3So IN THE COURSE (PASSAGE) OF TIME, Cain brought some of the fruit of the soil as an offering to the LORD, while Abel brought the best portions of the firstborn of his flock.
(Again perhaps you missed this issue of the passage of time, for Cain & Abel to be able to grow up; for Abel to have time to build up a HERD of sheep from his FLOCK & then time passing before he was able to offer the best portions (plural) of some of the firstborn (plural) that he gave as an offering to the LORD. Adam & Eve 'had other sons & daughters' over the passage of time.)
And the LORD looked with favor on Abel and his offering,
5but He had no regard for Cain & his offering. So Cain became very angry (wrathful) & his countenance fell.
6“Why are you angry,” said the LORD to Cain “and why has your countenance fallen? If you do what is right, will you not be accepted? But if you refuse to do what is right, sin is crouching at your door; it desires you, but you must master it.”
8Then Cain said to his brother Abel, “Let us go out to the field.” And while they were in the field, Cain rose up against his brother Abel & killed him. And the LORD said to Cain, “Where is your brother Abel?” “I do not know!” he answered. “Am I my brother’s keeper?”
10“What have you done?” replied the LORD. “The voice of your brother’s blood cries out to Me from the ground. Now you are cursed & banished from the ground, which has opened its mouth to receive your brother’s blood from your hand.
12When you till the ground, it will no longer yield its produce to you. You will be a FUGITIVE & a wanderer on the earth.”
13But Cain said to the LORD, “My punishment is greater than I can bear. Behold, this day You have driven me from the face of the earth & from Your face I will be hidden; I will be a fugitive & a wanderer on the earth & whoever finds me will kill me.”
(A fugitive is someone running away from doing something wrong)
15“Not so! ” replied the LORD. “If anyone slays Cain, then Cain will be avenged sevenfold.” And the LORD placed a mark on Cain, so that no one who found him would kill him. So Cain went out from the presence of the LORD & settled in the land of Nod (Heb. 'wandering'), east of Eden.
So we believe that what God gave Moses to write down SPECIFICALLY--IS THE TRUTH. Cain did have a fear someone might kill him because he was a FUGITIVE! The word had gone out what he had done & God's curse & mark on him. Lamech, one of Cain's descendants brought this out years later, when he killed someone. Also remember the long age that each person lived to back then. There would be quite a few people living DURING CAIN'S LIFETIME, that would cause such a fear.
Pseudo-Philo, a Jewish work of the 1st century AD, narrates that Cain murdered his brother at the age of 15. After escaping to the
Land of Nod, Cain fathered four sons: Enoch, Olad, Lizpha & Fosal & two daughters: Citha & Maac. Cain died at the age of 730, leaving his corrupt descendants spreading evil on the earth.
"According to the scripture him, Abel, Adam & Eve were literally the only people on the planet."
Again this is another assumption you are making, adding to the Word of God. You also forget that people back then lived between 900 & almost 1000 years old, had no DNA deficiencies built up as yet & lived in a disease-free environment for many years, until the population built up.
Just for your information, here is a very conservative estimate of the earth's population between Adam & Eve & the flood done by the well respected Christian apologist, Dr. Henry M. Morris, who has passed on into glory now.
Population of the PreFlood World
Third, you have MISSED how God is bringing forth His revelation PROGRESSIVELY & often with PINPOINT SPECIFICITY. In Chapter 1 for example, God FOCUSES on the GENERAL creation of the heavens & the earth. That ends at the beginning of Ch. 2. Many details are left out. Some things are difficult to understand with just this amount of information. Later revelation is revealed to fill in the gaps. It doesn't make it any less the TRUTH whether we can figure it out or not.
It is the BIG PICTURE VIEW. Yet God's progressive revelation CONTINUES beyond Genesis 1 concerning the DETAILS of His creating the heavens & the earth throughout the rest of Scripture, especially in Job & the Psalms.
Then God focuses on the creation of Adam & his GENERATIONS in Ch. 2-5, then moves on to the chapters concerning the global flood. Sometimes it is pinpoint specificity with Cain & Abel in the FIRST part of Ch. 4 & for a reason-- because in the progressive revelation of the rest of Scripture, they are key people mentioned again & again. Yet let us not miss the GENERAL issues of the generations of Adam in ALL the chapters of the generations of Adam combined.
The last part of Genesis Ch. 4 alone talks about Cain & his offspring & even building a city & some of his generations. Even one of those descendants, Lamech, killed someone also & says, 'If Cain is avenged sevenfold, then Lamech seventy-sevenfold.' So at THAT TIME, it was well known about God's curse & mark on Cain, WHO WAS STILL ALIVE at the time of the lives of his descendants.
And where did he get his wife? It becomes obvious from the whole testimony throughout Scripture that it had to be a daughter or a daughter of a daughter BECAUSE ADAM HAS OTHER SONS & DAUGHTERS & all mankind came from one man, Adam & populated the earth.
And it ends in vs 26 with: 'At that time mankind began to call upon the name of the LORD.' Whereas before God spoke directly to people, now all mankind began to pray TO GOD & to call upon Him in prayer. Again Scripture is progressive in revealment, whether specific or general.
Chapter 5 is PINPOINTING on Seth to start & for a reason. He is in the line of the promised Messiah & God specifically uses him to show the distinction between Adam & Eve being made in the image of God & now mankind being made in the image of Adam, as Romans 5 points out.
Then the chapter focuses on the GENERAL generations of Adam: 'he had other sons & daughters.' (5:4).
It does the same with Seth, SPECIFICALLY naming SOME of his offspring, then saying, 'he had other sons & daughters.' (5:6) Again Kenan, the son of Enosh (son of Seth), 'had other sons & daughters' (5:12) and so on. Cain's wife was one of these 'other daughters' of Adam & Eve.
For example, Eve is mentioned as the mother of all the living. (Gen 3:20--
Eve: The Mother of All the Living). So any offspring either were direct offspring of Eve during her lifetime or became descendants of Eve beyond her lifetime.
Paul said that all human beings in time came from one man, Adam.
Acts 17:24-27 The God who made the world & everything in it, being Lord of heaven & earth, does not live in temples made by man, nor is He served by human hands, as though He needed anything, since He Himself gives to all mankind life & breath & everything.
26And HE MADE FROM ONE MAN EVERY NATION OF MANKIND to live on all the face of the earth, having determined allotted periods & the boundaries of their dwelling place, that they should seek God & perhaps feel their way toward Him & find Him.
"Anyone that ever tries to answer them ends up making assumptions that are not backed by scripture--adding to the word of God."
I have already shown you have done the same thing in your judgments. You also have thrown out any & all explanations by limiting the answers & already 'predicting' no one will be able to give YOU answers. You have already shut the door to that by your own attitude & mindset.
'Anyone--that EVER TRIES--to answer them ends up making assumptions...'
"There are only two conclusions I can draw from this. Either the scripture is wrong or incomplete."
I have already shown your prior mis-assumptions & this is yet another one. The CANON of Scripture was decided thousands of years ago. God's revealed revelation, the Scriptures, is closed. Jesus Himself showed it was closed for the Old Testament (OT) AT HIS TIME! Yet you accuse others of adding to God's word and now you want to add spurious books long ago rejected as Scripture.
He gave the basic three-fold Jewish OT Scriptures: The Law, the Prophets & the Writings & called them Scripture. Another time He mentions the blood of the prophets down through history since the beginning, starting with Abel in Genesis & ending with Zechariah in 2 Chronicles. This prophet is mentioned in the LAST book of the Jewish OT. (in the Christian Bible it is Malachi, simply arranged differently)
I hope these are helpful to fill in the 'gaps' of your understanding as these have been taught since even the early days of Christianity.