It's far from clear to me what sin means, but I can tell you this with certainty: I have done my best all my life to be a good and decent man and to live a good and decent life, and the evidence strongly suggests I've made a pretty good job of it so far. And I've managed that without any belief in any god. I will never concede that I am a sinner in the sense you appear to mean. I don't share your beliefs, and probably never will, as you know, and frankly I resent being told I'm a sinner. I am not, in any way that makes sense to me.
1)Christ died to save all - Sacred Scripture tells us that. We also have free will; the ability to choose or reject God. Christ died to save us, but we don't have to accept his salvation; we can reject it (that is ultimately what sin is). Salvation is not automatic; it must be applied. If salvation were automatic, it would either over-ride free will, or become an oxymoron - it would have no meaning, because it was forced on us; but if it is forced on one who doesn't want it, it denies free will.
2) The concept of the "book of life" is another way of speaking of God's infinite and all encompassing knowledge. Because we are in time - everything we do is done in time - we think in terms of time. However, with God there is no time; all is simply present. If God were in time, then God would change, because time is a measure of change. But God is not changeable; God simply "is".
Therefore, all past, present and future simply is, to God; there is no before or after. Thus, it can be said that God knew from all time who would be saved; but that knowledge is different than ours, as we work our knowledge out minute by day by lifetime. Speaking of the book of life makes it seem that it has already occured (He knows the answer), but that implies that He is waiting for the play of life to play itself out. But He isn't, because there is not time with God.
God is not outside of time waiting for it to finish; God is simply outside of time. We still have free will; we can still accept or reject God; we won't know the results until we are all done and judged, but we make choices all along the way.