Can I start with the low hurdles?
There are an awful lot of conditions that need fulfilled here to make you happy.
Off the top of my head I can think of the Woman at the well, the thief on the cross, the centurion, the woman with the issue of blood, the woman caught in adultery, the guy up in the tree, Thomas who poked around the Truth, and the blessed folks who have not seen.
That's what I asked you to find. The conditions that I precisely gave you are those that the scripture declares necessary to qualify for receiving eternal life. Genuineness. (If you can lose it, it was not eternal, was it?)
No works involved. Only the omniscient heart-searching God can discern that state of mind of the "believer," as to whether hearing the Gospel preached faithfully, admits to him/herself of being spiritually dead in sins and unable to "save" oneself out of God's condemnation, and desires with his/her whole heart to be made God's obedient child with sin-debt cancelled and never-ending life in Jesus under His rule the benefit.
Where does the scripture give an instance of someone that having received this gift and experiencing the newness of it, has actually decided to change his/her mind again, and thus reject and abandon this gift?
The examples you named do not fit this description of turning to God for salvation from sin and sinning, receiving birth in the spirit, then deciding to reject God, Jesus, the new life, its benefits, and to return to a life of sinning.
See 2 Peter 2:9-22 for those who spent some time with saved people and the Christian culture, but were NOT saved, finally returning to their old ways.