Why couldn't we have this magical inability, immunity, whatever you want to call it in the beginning of creation? If it doesn't violate free will as you say it does then why wouldn't God want us to have it already in us? Wouldn't this have been a big advantage to have?
But evil had to have had a foothold initially right? What was the driving force for Satan and other angels to disobey a loving Creator who satisfied them completely? If angels were susceptible to evil in Heaven why won't you be just as susceptible as they were? There isn't really a clear-cut response from you on this.
And why is this exactly? One man sins and the rest of humanity has to pay the price for it. Angels sin and it only affects themselves not other angels. Why should I have to forfeit my initial relationship with God because of something someone else did?
I don't think you're going to get a clear-cut response here, because we're not given a clear-cut response in scripture. We're told fairly early on in the bible that there are things that we just don't need to know:
Deuteronomy 29 (NIV) said:
[29]The secret things belong to the LORD our God, but the things revealed belong to us and to our children forever, that we may follow all the words of this law.
We can speculate, based on the small glimpses of information we are given about the nature if angels and heaven we do see in scripture, but there's really just not enough definitive information given to us to make any concrete conclusions. Remember, the bible was meant to be a user guide for this world, not the next.
Given this, what I was trying to provide we're scenarios that might could possibly explain the apparent contradictions which you seem to be perceiving regarding sin in heaven. I of course don't know for certain and I would be cautious about anyone who said they did for certain.
What I was trying to say about time in heaven was that if in heaven we experience time as God does, than there would be no "initially", no now, no two days from now. God is not bound to time linearly as we are. He experiences all time, past, present and future as a single instant. He doesn't remember Adam's fall in the garden, he's there right now, and at the same time, he's there the moment that you were born, and at the same time there the moment Lucifer is cast out of heaven, and at the same time there the moment that anything we perceive to be in the future will occur. If there is no time in heaven then it seems reasonable to me that there would be no ongoing battle between the urge to sin and the urge to not sin as we experience during our earthy existence. E.g. I conquer the temptation to engage in a particular sin today, but tomorrow, or the next time I turn on my computer, or the next time I see a particular person, the temptation is right there again. It may be that there is no next time in heaven. In heaven a single choice may be sufficient for all of eternity. So it's possible that the very choice that permits me into the Kingdom of heaven, is the same choice which prevents me from sinning for all of eternity.
Angels don't really have a bloodline to be corrupted by sin as man does, but their punishment was, in my opinion, far more permanent then man's. There is no hope of salvation through Christ for the fallen angels. They made their choice, their "election":
1 Timothy 5 (NIV) said:
[21]I charge you, in the sight of God and Christ Jesus and the elect angels, to keep these instructions without partiality, and to do nothing out of favoritism.
and once they did, their eternal fates sealed. We, on the other hand, still have the choice. We live in a world corrupted by sin, and the natural effect of that sin is death, but we are offered an antidote in Christ's sacrifice. We may choose to take that antidote or suffer the consequence of not taking it.
Don't think of original sin as us being punished for the sin of Adam and Eve. Think of it like living in a closed environment like a ship or a geo-dome and someone allowing the entry of a deadly virus. Before the introduction of the virus, there was no threat of suffering, sickness and death, but now that it's become part of the environment, everyone must deal with the effects of the virus. It's like that with sin, before sin was allowed into the world, there was no consequence of sin, but once it was, we all must deal with the consequence. It's not magical, it's just that you don't get sick in an environment where there are no germs. I suppose God could have made us in a way that we were not susceptible to sin, but that would violate free will; sort of like getting the girl you like to marry you by holding a gun to her head or marrying a department store mannequin because she cannot choose to not love you. It might look like love, but it's not love. What God wants of us is obedience through love, not obedience regardless of love.
Again, I don't know that any of this is right. It's what make sense to me but to you or someone else, it may seem like nonsense. And it's ok if it does, because as Christians, we don't have to agree on every specific detail about which we are not given specific information in God's word. It's ok for us to have opinions about such things so long as those opinions are not contrary to God's word.