1. Satan's similarity to us
You wrote: "...he was a Holy Angel in Heaven and through disobedience was cast down. Just like we are removed from the Book of Life for unrepentant disobedience."
The two cases are completely different and dissimilar, not "just like." Satan and the other demons were created sinless in heaven and were later cast out after rebelling. We are created sinful here, a way-station to our ultimate destination. We all rebel from the womb, from the instant of our creation ("Surely I was sinful at birth, sinful from the time my mother conceived me." Ps. 51:5). But we cannot save ourselves. God saves us by grace and, once saved, He keeps us saved. So just as we cannot save ourselves, so also we cannot "unsave" ourselves, because He is operating in and for us through His Spirit. He invests Himself in us and actively prevents us from renouncing our salvation. (I strongly encourage you to memorize Ephesians 2:8-9 and then meditate on it frequently--as often as the Spirit puts it in your conscious mind--for a year or so, allowing the Holy Spirit to embed and bring alive its vital and encouraging truth in you.)
2. Age of Accountability
I find no scriptural warrant for this strange but popular concept. As above, our spirits rebel from conception onward, and God is able to communicate even with the spirits of fetuses. Since He forces heaven upon no one, if they die in utero or as young children they still will have had the opportunity to make an informed choice of their eternal home and eternal lord.
Soteriology of the Old Testament is also an interesting thing. There were persons who could say, as in the Psalm, that afterward God would receive them unto glory.
This seems to imply some kind of private Holy Spirit ministry, which coexists uneasily with the scripture about a preacher being needed, but again in the extreme instance the Lord can personally be that preacher. He engages men too in that role not because He is logically forced to, but because in love He desires their involvement. We have left legalism and stepped into love, which in the end achieves everything that the law stated was necessary.
Sorry life moves. Please look at the sower of seed story Luke 8:4-21 the seed that fell and was eaten by the birds represents people that don’t accept the Gospel, but the remainder have accepted the Good News. Some don’t last long at all some a little longer and well some produce fruit. Again please don’t think that I’m saying that the Devil can pluck you out of God’s hands, but to say that once saved all behavior of unrepentant sin will be forgiven is just wrong. God Bless