Yes indeed, there is a tension. I did list a short list of 10 verses that distinctly point to the potential of going astray, along with 6 verses assuring our salvation. That assurance is largely dependent and reflect on the 10+ verses that warn about forsaking the faith. Taking all the New Testament in context, we come away with an assurance on God's part, but dependence of sustaining on our part in combination with continuing in the Holy Spirit. We don't have a prayer to remain on the path if we try in the flesh, but must continue in the Word, to DAILY purge the flesh and enlarge the Spirit.

There is no formula to apply as to ascertain the exact outcome for each person. Where the Lord did respond to the disciples to the question, "Then who can be saved?" But He said, "The things that are impossible with people are possible with God." If we are flirting with apostasy, then we increase our risks of abandoning the way. If we are embracing sin or courting the world without limit, we are certainly revealing a growing apostate condition that is prone to abrupt departure.

My modification of the saying has always been "Once Saved Always Saved If We Are Saved." Not perfect theology, but the implication is if we are truly in Christ, then we will remain in Christ. If we are in the faith for any other reason than genuinely being born again, we will not remain true to the course, but wander away. Thus the parable of the four types of soil in Matthew 13 and Mark 4, all showed signs of change, some for a short time, others for the long time, but only the fourth was genuinely transformed in heart and deed and remained true to the course and inherited eternal life. Or again the parable about the man who hid his inheritance. He had a form religion, was not questioned by any in attendance, but was not part of the family, "And throw that worthless servant outside, into the darkness, where there will be weeping and gnashing of teeth." Matt. 25:30 Therefore the idea of hopefully sliding in by a narrow margin may be a dangerous premise, where such could be found disqualified.

I don't have any books in my library that I could recommend off hand. Most my studies come from years of digging, as you, seeking the truth until I could come away with a balanced and contextualed answer.