It's interesting. You'll discover that your investigators can come up with all sorts of questions, and in the end, it all comes back to the fundamentals. 2 Nephi 2:22-23
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.
And they would have had no children; wherefore they would have remained in a state of innocence, having no joy, for they knew no misery; doing no good, for they knew no sin.
So, in the end, it all comes down to the Book of Mormon. Is the Book of Mormon an inspired book of scripture given by revelation? If it is, then the concern is resolved; they couldn't
have had children, and the Fall was necessary for the progress of mankind.
It's important to understand that the Gospel is a package deal. Nobody can simply accept the parts of the Gospel with which they agree. If Joseph Smith was really a prophet, and he really translated the Book of Mormon from ancient writings through the power of God, then everything that follows from that is true. If he wasn't a prophet, then nothing else we teach has any greater claim to validity than anyone else's doctrine. Help your investigator understand that the specific questions are all answered by the Gospel, but only
if the Gospel really was restored by Joseph Smith. That's the crux of everything.
As you're preparing for your own mission, remember that. Some missionaries get caught up in trying to resolve concerns principally from the Bible, and others try to resolve concerns with their own personal testimony tying it to the Restoration. That is our message. The Gospel of Jesus Christ has been restored by divine revelation. Everything else we have to say is basically irrelevant if that's not true.
Remember that. Everything comes back to the Restoration. Our understanding of Jesus Christ, his doctrines, his life, his atonement, everything
comes back to the Restoration. Learn to tie things back to the Restoration, and you'll be a powerful missionary.