In the beginning, the ten commandments were just 2. "Do not eat of this tree, nor this tree." Even with just 2, we couldn't obey.
While humanity may not yet have had the knowledge of good and evil, they did have the ability to understand the word, "No," so no excuse for "lack of knowledge."
However, if you read carefully, you see that the one deceived, Eve, had not been told by God not to eat the fruit, she had been told by her husband. Therefore, as he had the word from God, His sin was accounted greater than hers.
As for God calling them, as John tells us that Jesus was always with God, I think that it was Jesus who walked in the Garden with them...especially as God tells us later that no one can see His face and survive.