What I asked is this:
IF the sin nature is eliminated at salvation....
and it is the sin nature that causes man to sin...
then why does man sin after salvation if the sin nature has been eradicated?
I haven't had my coffee yet, so I'll finish reading your post later, but I can answer this one first.
John 15:3 is about the old sin nature that Jesus as the Author of our faith handles first. It says, "you are clean." That is because in 1 John 3:4-5 Jesus takes away lawlessness, John calls "sins unto death" and the Catholic Church had right calling them "mortal sins." The next verse in John 15 has to do now with the maturing of the fruit of the Spirit. Because not every fruit is mature at the beginning of our walk on the path towards salvation, we will unintentionally commit the lesser type of sin called "sins not unto death." Jesus does the work as the Finisher of our faith, and all we have to do is love Him and keep His commandments, which is easy with the indwelling Holy Spirit we received when and if we became born again of the Spirit. (I told you, it was 30 years before I was born again even though I went to church all that time.)
So, yes, we will "sin" after being born again, but it is imperative to know the difference between the two types of sin, which the Reformation botched up lumping them all together. Were you ever taught their heresy that "sin is sin"? Many have. I wasn't, thank God. But a Christian WILL NOT commit sins unto death (the willful breaking of a commandment of God) so 1 John 1:7 is NOT speaking of that type of sin as the Reformationists taught. It says, "If you walk in the Light, as Jesus is in the Light, you will have fellowship with one another, AND THE BLOOD OF JESUS WILL CLEANSE YOU OF ALL SIN." Can you see why Martin Luther confused the two and didn't know the heresy he was proclaiming when he wrote "that even if you committed murder and adultery (both willful sins unto death) a thousand times a day, it will not separate you from God." 1 John 1:7 was talking of the only sins a born again Christian is even capable of unintentionally committing and those are from immature fruit of the Spirit that our Finisher of our faith is continually ripening until as Peter says about these when Jesus has completed His work, we will "never stumble." 2 Peter 1:5-11.