Adam's immortality was apparently something that was renewed by eating the tree of life, not because time had not begun. What you say means that God's creation didn't really happen, or take effect, until the fall. When God said 'very good' he was talking about a highly simplified and temporary condition, in your view, and real life in all its wonder and complexity only came about because of sin. I think that has worse theological implications than there being animal death before human sin (it probably wasn't before Satan's sin). Romans 5 only talks about human death. I recognize it doesn't seem as perfect to have animal death as part of the original creation, and that kept me from accepting an old earth for some time, but that is a value judgment we can leave to God. He said it was very good, not perfect, and I believe the natural world is still very good. Besides, Satan may have played a role in marring it even before man came on the scene. A perfect Eden would not have had the serpent in it, for instance.