God did not "have to" anything.. He is God he is not limited by anything.
Yes! Now you're getting somewhere. This is an answer to your earlier question about the incarnation. Good.
What was this monstruos act that offended God so much as to punish the entire future humanity for it?
I don't know if that's the right way to look at it. Our sin causes us to become distant from God by our own acts, and we inherited a propensity towards sin by virtue of being born into a humanity that is affected by the fall, but it's not like God is sitting up in heaven, stewing and being mad and plotting how to send us all to hell for being naughty. We're not Calvinists.
So we do need a savior, but from the wages of our sin (the wages of sin being death, you'll recall), not from "angry dad" God.
The Bible recalls other people defeating death and sin.
No, no...not "defeating". Like we believe that St. Mary was personally sinless (or at least that is my understanding of the topic; I am new to Orthodoxy), but she did not herself defeat
sin and death. Sin and death were still a problem, and she could not save anyone from them. Only God can, and only God (Jesus Christ, the Son) did.
You said that because of Jesus(and his resurrection) we all have the capacity to accept his gift and eternal life, presuming that we did not have this capacity before.. This automatically interfears with another one of your sayings that God gave us free-will.
How can these conflict when before Christ and His glorious resurrection, we didn't have
eternal life? "...that whoever believes in Him shall not perish, but have eternal life", right? The gift was not even there yet. I thought that was the whole point of Christ's descent into Hades, because it is recognized that there are those who died before the coming of Christ who likewise need His gift.
About the "free" party.. Yes, it is your fault.When something is conditional it is no longer free.And sometimes the cheapest and freest things cost us the most, as a certain saying goes.. And when a gift is conditional it ceases to be a gift.
Maybe I am the one who is confused here, but it doesn't seem that you've understood Mabsoota's example. To put conditions on something does not make it no longer free. Something is only free when you don't have to PAY for it (that's what "free" means, in this sense; think about the difference between the words "gratuit" and "liber"*), not because there are no conditions. It is maybe a bit like receiving a rebate in the mail after buying a particular product. Generally to get those, you have to mail in the "proof of purchase". Nobody would consider this an unreasonable condition relative to the return, and I would hope that any Christian would see the parallel with Christianity here: You certainly can't just do NOTHING ("Once Saved, Always Saved"/"Eternal Security" is not a correct doctrine), but if you do what is asked of you as a follower of Christ, then your reward is great (in fact, my analogy is pretty poor because it's so very great; it'd be like mailing in a rebate for $5 and receiving more wealth than can ever be imagined in return).
* - I do not speak Romanian, but these are the Google translations of the two Spanish words (a language I do speak) I was thinking of, "gratis" and "libre".