Hi NV, even if the consumption of the apple (perhaps of a particular enzyme in it, or something like that) was not required to cause the moral change that occurred in our first parents (and I, personally, do not believe it was), the tree was still
necessary (or something else like it would have been) to test whether Adam and Eve would choose to obey or disobey a direct commandment from God.
Look, the Bible is a wonderful book. We believe it contains everything we need to know God, to find Him, to obey Him, to please Him, to love Him, to worship Him, and eventually, to be with Him face to face. But while that's true, that the Bible is great at telling us everything we
need to know, it hardly tell us everything that can be known about most of the subjects it discusses.
Such is the case in the opening chapters of
Genesis. We can figure out what basically happened there, but the information we have is limited. IOW, we are not told
all of the ins and outs of what went on.
There are also things which I believe we could understand but, for whatever reason, God decided not to reveal to us right now. And there are things which have been revealed to us already that we clearly have no capacity in our present state to fully comprehend, like the Trinity, the dual nature of Christ, how God can exist everywhere (including the past/present/future) at the same time, know everything, do anything that is possible to do (He cannot make a round triangle or 2 + 2 = 5 however

), or create the universe ex nihilo, etc.
We are also told that there things which remain known only to God, the "
secret things" (see
Deuteronomy 29:29). But again, the Bible makes it clear that all we
need to know for now has been revealed to us
My point is, we are doing our best to answer your questions based on the information we have available to us right now. But you are asking for answers to questions that go beyond the data we have available at our fingertips, so to speak, so some of this becomes conjecture on our parts (our best guess based upon various criteria such as the information in the Bible, what we know about God and His character, etc.)
Take the "Tree of Life" for instance. What did it do? It somehow made eternal life possible. That's what we know. Does that mean physical and spiritual life? We aren't told. Do the properties of that fruit cause our bodies to be free of sickness and degradation forever, or would it have "locked in" our first parents' moral choice forever (
had they made the right choice to obey instead of disobey, of course)? Again, we aren't given those details.
So we know that the trees existed and what their general purpose was, but how it all works, that we really aren't told.
And that's ok 
In fact, I would say that God knew exactly what He was doing because the information He gives us causes us to focus more of our attention towards what really matters. IOW, by giving us what He gave us information-wise, we are forced to major on the majors, rather than the minors (even though the stuff that we know less about is often that which piques our curiosity the most, isn't it

).
Fortunately for us, God is wise
Yours and His,
David