There was indeed no death before the Fall as is taught. However, in 2 Nephi 22 we see that a state of no death is NOT applied to the creative state before all was ready and placed into the garden. So death can exist prior to the garden state because prior to the garden state is undefined:
And now, behold, if Adam had not transgressed he would not have fallen, but he would have remained in the garden of Eden. And all things which were created must have remained in the same state in which they were after they were created; and they must have remained forever, and had no end. 2 Nephi 2:22
Careful BC. This is YOUR spin on this passage and is not official LDS doctrine. It is only published by you. And on top of that you sure need to read a lot into the verse to reach your far out conclusion.
Depends on local or global garden state which seems to be undefined doctrinally. If local, then it is probably figurative in the sense that they are the first to Fall.
The Church officially teaches we all descended from Adam and Eve, and after the flood Noah and his family.
The Church does not teach that any human alive today is descended from some other humanoid pre Adamite non spirit child of God living thing.