both of them. the serpent tempted Eve, and she went and told Adam to try it, and he sinned as well. if anything, Adam is more guilty. he wasn't purposely tricked by the devil himself. Eve pretty much went up to him and said "Hey honey, try this! it's delicious!" see my point?
There is no such thing as the original sin. Adam and Eve both ate from the tree and they both ask for forgiveness. That's it! God does not hold me responsible for Adam because Adam didnt ask me permission to eat from the tree nor did Eve asked my wife.
Well I wouldn't know fo sure. But in today's court they could considered "insane" (not knowing right from wrong). If they existed they probably didn't know about Satan, lying, evil. They would be too easy to manipulate. I feel more sorry for them then I would angry or dissappinted in them. Imagine living a life of no care or pain then suddenly being kicked out of your heavenly home and have all the pains and truth of the world thrown at you.