You have a good concern, but I think that Mr. Comfort’s omission to “acknowledge on his part that the 93% fall away because they were never really saved in the first place” was not because of a doctrinal error, but just a different focus. In his book written with Kirk Cameron, “The Way of the Master” he does call them false converts. His main point though is against a diluted message of salvation, which does not mention the law (the “God loves you and has a wonderful plan for your life” stuff). In his book he quotes A.W.Tozer who wrote: “It is my opinion that tens of thousands of people, if not millions, have been brought into some kind of religious experience by accepting Christ, and they have not been saved.” I think he also touches on the second part of your concern- about why they “choose to stay”: he points out as the reason to why so many who professed to make a decision fall away- “In light of the fact that “Now unto him that is able to keep you from falling, and to present you faultless before the presence of his glory with exceeding joy, Jud 1:24”, either He wasn’t able to keep them, or His hand wasn’t in their profession of faith in the first place.”

And now on to a tangent on apologetics and "proof vs evidence": The two main branches as I understand it are evidentialism and Presuppositionalism. I believe they can both be excellent tools for sharing the gospel, but that they should still be used as minor tools alongside the Great tool of Gods word and His Law. Note what Paul says:
1Co 1:17 For Christ sent me not to baptize, but to preach the gospel: not with wisdom of words, lest the cross of Christ should be made of none effect.
1Co 1:18 For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.
1Co 1:19 For it is written, I will destroy the wisdom of the wise, and will bring to nothing the understanding of the prudent.
1Co 1:20 Where is the wise? where is the scribe? where is the disputer of this world? hath not God made foolish the wisdom of this world?
1Co 1:21 For after that in the wisdom of God the world by wisdom knew not God, it pleased God by the foolishness of preaching to save them that believe.
1Co 1:22 For the Jews require a sign, and the Greeks seek after wisdom:
1Co 1:23 But we preach Christ crucified…

The Jews require a sign…evidence …Greeks seek after wisdom… the presuppositional method- Interesting.
Spurgeon warns: Another very great obstacle to soul-winning is unbelief. …These people are very hard to influence; but I would warn you not to fight them with their own weapons. I do not believe that infidels ever are won by argument; or, if so, it very seldom happens. The argument that convinces men of the reality of religion, is that which they gather from the holiness and earnestness of those who profess to be Christ's followers. As a rule, they barricade their minds against the assaults of reason…

I had a friend at school who kept asking me to “prove Christianity” – but when pressed as I told him that I could give him “evidences” – he replied that he really did not want to believe. So much for his claim to be seeking the truth. (He is a philosophy major) and he had barricaded his mind against the assaults of reason.
i.e it is the Holy Spirit that works in someones heart to bring them to a saving knowledge; trying to "convince someone" often doesn't work even when we tell them that we have "evidence that demands a verdict."
Let us then, by the foolishness of preaching to save them that believe.