Brother John,
I do not want you to think that I am avoiding your question by the answers that I give. I will tell you first what I believe in regards to unbelievers and defending scripture, what my response will be to them, and then I will tell you what my response will be to a professing believer.
First, our beliefs about Scripture is a doctrinal issue. It is a doctrinal issue because the Scripture says a lot about itself in regards to its usefulness, purpose, inspiration, trustworthiness, etc. When I encounter unbelievers, I make it a point to preach nothing but Christ and him crucified. There is a reason for this:
The primary reason is because the enemy is cunning. When a man is preaching the Gospel, the enemy knows that he cannot simply make a beautiful girl walk by and totally distract him from preaching the gospel. He knows that he cannot simply react in violence against the preacher because it will only make him respond with joy and more preaching of the gospel. I am probably going to be in trouble for saying this, but the primary method, I believe, that satan uses against us to prevent us from preaching the gospel accurately is doctrine. Christians should love doctrine. They should live doctrine. They should also proclaim the Gospel (which has some wonderful theological points), and it is the preaching of the gospel that allows men to hear the Word, be drawn by the Father, and believing, become saved. However, not all our doctrines as believers are the gospel--this includes the trustworthiness of the scriptures. Satan will distract us from preaching the Gospel by making us defend the Scriptures. If someone said to you, "prove to me that Jesus is the son of God!" or "Prove to me that Jesus turned water into wine!" Now we move from preaching the Gospel to teaching something else, which can be very dangerous. I make it a point not to debate with lost people about these things because I have never successfully proclaimed the Gospel when I try to also be an apologist. When I was lost, all my objections and questions and wrong-headed ideas melted away (or at least started to) when I heard the Gospel, believed it, and came to know the Lord. They just started dropping off. Really, the world would consider me a fool, but I don't care anymore about what they think.
In regards to beleivers:
Apologetics is very useful. But we must remember, the burden is not on us to prove that what we say is true, but for them to prove that what we say is false. We should take another believers objections and answer them prayerfully, biblically, and with good, solid research. We should also make a clear distinction between minor issues and major issues. Not every issue that a believer objects to is a potentially faith-breaking, soul-crushing issue. So we need to make sure that the brother is not treating something like a comma-placement or a mispelling like something that would totally invalidates the atonement of Jesus Christ. You see what I mean?
Also, I recommend doing research together, praying, reading, researching, etc. Because, when you and the brother get through this, together, you will be much closer to the Lord and to one another, which is one of the goals of the church in the first place and is definitely what part of discipleship is all about.
Also, this is a wonderful website:
http://www.greatsite.com/timeline-english-bible-history/
It is a fantastic, thoroughly research, resource in regards to the Scriptures. I think you will find it very useful.
I know I didn't answer your question directly, but there is no way I could answer the entirety of objectives on this site. We just take them one step at a time.
A final word: If a professing believer rejects or begins to reject the faith, we need to follow matthew 18 and preach the gospel to them. The gospel is a powerful reminder of why we believe what we believe. And if the person is actually an unbeliever, masking as a Christian, then it will give them an opportunity to be saved. But if the Lord lays it plainly on you that the person you are dealing with is lost, you need to cease trying to prove the validity of the Scriptures and start preaching the Gospel, because if they are lost, they have no spiritual foundation for which to confirm that anything you say is true.