What we know about Eden doesn't seem to square with what you seem to be assuming about Eden. Yes, it existed before sin, but what does that have to do with anything?
We are told little about Eden. The word usually designates a park-like setting featuring landscaping. Notice that the "park" is planted *in Eden*. Here we have a park *in Eden*. (The name "Eden" comes from an Aramaic cognate meaning "abundance.") Generally in ancient days, gardens (parks) were planted next to temples (the god's residence). The Genesis text describes a situation that was well known in the ancient world: a sacred spot featuring a spring with an adjoining, well-watered park, stocked with specimens of trees and animals.
It was a place where priests and kings could meet with the deity. No one lived there (neither the people nor the god); it's where they met with the god. This is exactly what we see in Genesis: the humans (God's priest and priestess) came to the park to meet with God. It's an archetypal (notice I didn't say metaphor or allegory) sanctuary. It's more sacred space than green space. It is the center of order, not perfection, and its significance has more to do with divine presence than a human paradise. We don't read that the Garden was paradise, nor a place of blissful enjoyment. Instead, the man and woman come there to meet with God.
The true question is, "Why is this a problem that the man and woman first sinned there?" Temptation can exist anywhere, including in sacred space.
Let's examine your premises:
* "Eating X causes Y." In this case, presumably, eating the fruit of the tree of knowledge of G&E causes sin and death. OK.
* "Y is required to eat X." Sin and death is required to eat fruit? No, this doesn't make sense.
* "Z doesn't have Y." I don't know what "Z" represents, but you probably mean the man and the woman don't have "sin and death." You're right; they have not yet sinned.
* "Therefore, Z can't eat X." Therefore the man & woman can't eat the fruit? This doesn't follow. It looks like your second premise is where the argument falls apart.