lol. XD well, it's a good thing you were pretty detailed in expounding on your fic, else i really would go to "well, it all depends" mode.
i write Bible fanfics because it's just the same (for me) with making a movie out of things from the Bible, like the Passion of the Christ, for one. my thinking is that it's my interpretation of what happened in the Bible and as far as i'm concerned, movies, books, songs that were inspired by the Bible as a source are in that category too. it's the writer's interpretation.
however, my fics work as retelling Bible stories in a very high degree. i retain everything and just add more dialogue, socio-historic background...stuff like that. like a movie. it's still the same Bible story, with just more embellishments.
erm, yours is sort of different because it takes a "what-if" from the Bible and everything else will depend on your own theology. it's not so objective as a retelling. so you will have to be very, very careful because the content you write might be very controversial. i think it's ok for you to do it...after all, exploring what you believe in is best done in writing because writing takes a lot of organization and understanding to make it coherent. but i will advise you to do a lot of research and make sure that if someone will challenge you (and i'm sure somenoe will) regarding your POV as to what might have happened if Adam and Eve didn't eat the fruit, you should be able to stand up for it, if you do believe in it truly.
it's really very dangerous ground. ^^;; and it's a very ambitious proposal. lol, i wouldn't do it, but if you can, then more power to you.