The book of Genesis is definitely considered allegorical by most Christians, including the Pope. However most Christians also believe that left to themselves, humans quickly descent into sin, and from there war, pestilence, famine and whatnot. Jesus saves us not because he died on the cross, that is just a spectacular example of incomprehensible self-sacrifice. He saves us because if you believe in him, then you will not descend into sin, simply because by loving your neighbor, war, famine, whatnot becomes quickly impossible.
Anyway, even if the garden of Eden never happened, Christianity does not collapse. Christianity is a faith, it can explain away anything.
As Gandhi said, I love your Christ but I don't love your Christians.