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Hey Pablocito, you have probably been at studying scripture for more time than me and possibly lived more in line with the way we should be living and possibly have always been held in greater esteem than me. You refer to a couple of verses but Paul, I believe, states that all scripture is useful. Of course at that time he was probably talking OT and maybe they had some other books of which we are not aware today? I think you need to redo some of the things you write here. I have now done a pre written something which follows if you want read it.
I read the above 1st paragraph of your post and will the read the rest after I dealt with this first part. You have probably read a 1000 times more of the bible than I have.
My history is that I am not a reader and basically failed most of my English classes. This probably was my major excuse for not studying the bible before 2018.
Now even when I started in 2018 I met a roadblock because I am a very slow reader, I read the bible as though I was tackling a mathematics or computer problem. So I really never read the bible. God somehow did a miracle and moved me around the bible from one place to another confirming many things to me, that really blew my mind.
The problem with the bible is that almost everybody (Christians, or so-called Christians have a piece of it) In some sense they are all right because what they are practicing comes straight out of the bible. For example the Seventh Day Adventist goes back to the Sabbath, which, yes, God would kill the man who broke it. The Pentecostals go to Day of Pentecost and yes that was the most important day of Christianity in the 1st Century. You have the Calvinist who have it right when they pull from Paul's epistles their unique doctrines of predestination, and the list goes on and on to hundreds of different sects.
Now believe it or not truth (or what some people call fact) is really stranger than fiction. God has not given us in these last days any particular path to follow. Each man is on his own. What God has given to us is his complete word the bible with nothing more to add to it and warning that nothing should be taken away from it.
Each man takes what he wants. That is man's way. But God has laid down the ground rules, and if the ground rules are ignored, man will eventually be led into making his own ground rules and thus ending up with what he deems important.
The truth is the for the people of these last days, it is the "word of God" that will either make us or break us. This is the only generation where doctrine is "king", and will determine "heaven" or "hell" for all people, who are called of God.
The Holy Ghost, called the Spirit of Truth is the only guide that can interpret the bible. It is so complex, even if you lived for a gazillion years, no one whether Paul, or David or Peter or anyone else could understand the bible, if they were living today, unless they were guided by the Spirit of truth. (The Spirit of Truth is God's gatekeeper into his Kingdom.)
Ultimately what it means, is that if we know God, then we have his Spirit of truth within us, but really we have to get to know God first because we cannot do it with our own academic devices and listening to the experts (those are the worst people), because God tells those people nothing.
I cannot put into words or even give you a formula for how anyone should approach their bible study, as God is infinite and probably uses a donkey to talk to one of his prophets, Balaam, while he uses the wisest man, Solomon to guide others into the right paths, and even uses Paul a so-called intellectual to lead the gentiles into his salvation.
The only thing I know for sure is that when you are receiving truth from the Holy Spirit, you will know without a shadow of a doubt, in other words, you will know it better than you know your own blood brother or sister or mother.
I know you have serious problems with faith as you mentioned that there could be some other books that are part of the bible that we do not know about. Without faith God will not give you anything. It means that you cannot recognize God's voice and ultimately are using your own man-made wisdom.
I don't know what else to say, but I can only encourage you to take the bible seriously and believe that God is sovereign and in complete control and that Satan is is no god and cannot thwart God in any way or fashion.
I will read the rest of your post by tomorrow.