Firstly, I've showed that as natural men we cannot receive it by ourselves at any stretch (Matt.19:24-26, John 6:28-29,44, Eph.2:1-3, 1Cor.12:14), so we cannot take it, it has to be given to us (Matt.19:11, Mark 4:11, Luke 8:10). Once you see this, you should understand that salvation is wholly of God and we have actually no "part to play" in it, because it is not of us, it is of God alone. This is a principle we must hold on to throughout reading all scripture. Secondly, you say that you "totally believe" we are saved by grace, yet you also say that we can go out of that grace. That has to mean that you either believe that grace is earned and have to be maintained by us continually earning and "choosing" it else we go out on it. Or it means that the grace spoken of here is not really saving in itself at all, it just gives us the chance to work a little better on getting saved, if lucky enough. I can't agree with this since it is unscriptural.
Rev.3:5: This verse does not suggest a possibility of the loss of salvation, for those who are saved. Those who are saved, the Elect, have their names written in the book of life from the foundation of the world and it will remain there (Rev.13:8,17:8). Just as their election and salvation was likewise secured from the foundation of the world by the will of God and not their merit (John 17:4, Rom.9:11, Eph.1.4, Heb.4:3). However, scripture does indeed use terms such being cut off and falling away, but this always refers to the tare, goats or weed. Jude said that such souls had "crept in unawares", souls that God had "of old" ordained to condemnation (Jude v.4). These people were professing believers, they thought that they both did great things for God and experienced great things of God, but they NEVER belonged to Him. They were never saved (Matt.7:21-23). Those who "overcome" do so solely on the ground of the "blood of the lamb" (Rev.7:14,12:11) - not because of anything they add to this work.
James 5:19-20: The greek word used here does not mean convert as in convert to save, but convert as to turn someone from an erring or heretic belief to a correct belief. Another example of this is found in Gal.6:1-2. To think that a work of man can save another man's soul from hell is a very unsystematical interpretation of this scripture. If one is systematically interpreting this scripture with the knowledge that only the work of Christ is what makes the difference between heaven and hell for a sinner it is impossible to think that what is spoken of here should have anything to do with salvation being in the hands of men.
1Cor 9:27: The word "castaway" must not explicitly be referring to salvation here. Paul was concerned about becoming ineffective in his ministry, that his service would be rejected at the judgment seat. Paul was not worried that he could be lost. A contextual interpretation makes this plain since in the same epistle he taught that Christ preserves the believer (1Cor.1:7-9). His concern was about falling short of God’s calling for his life. He mentions running a race and winning the prize. To confuse this passage with salvation must stem from a misunderstanding of the gospel. Salvation is not a reward for faithful and excellent service. The Bible is clear on that salvation is by the free and unmerited grace and mercy of God. Anything that is merited or rewarded is not saving grace (Rom.11:6). But once we are saved we are called to serve Christ and doing so well means a "reward". If a believer on the other hand is not doing this well, he will be chastened by God (Heb.12:6-8, 1Cor.11:31-32). But even if his service was not acceptable, he shall still be saved, even if it has to be by fire (1Cor.3:11-15).
We are not to lean to our own understanding (and we can not know all things). However, the enlightenment of the Word of God is external as well as internal, equally affecting heart and mind/intellect. If we do not see this, we have a problem. The unsaved cannot "read and understand" because the Word of God is foolishness to them, but that does not mean it is impossible for the saved to read and understand. We have to have due respect for the revelation of the Holy Spirit in the Word - and not to put or seek that somewhere else. That's why letting scripture interpret scripture is so very important. The Holy Spirit would not reveal anything that is contrary to scripture, nor to its historical facts.
I agree with you that (most) people will believe what they want to believe anyway, however I see no problem with wanting to believe that which is true, but I do see a big problem with people "choosing" to believe something because of their own (or others') mere preference or "feelings" about that something. Usually such people have a very hard time seeing scripture objectively. So, if they are in error it is usually very hard for them to be convinced of it unless they have a certain subjective feeling or (internal) experience. If one has an all too fixed idea of how the Holy Spirit works and directs, one can easily fall prey to a self-centered and subjective approach to scripture reading/interpretation that is certain to take one off base. The question is only how much.
Finally I add that I don't strictly believe in what is sometimes called OSAS (once saved always saved) but I believe in what is known as the perseverance of the Saints (P of TULIP). If you want to know more about that (or something else) just ask.