I am at a loss to understand "authorities" (not General Authorities), who publish paper after paper, book after book on their "research" as to where the Book of Mormon supposedly takes place and who the descendents of the Book of Mormon people are.
First of all, we are told in the Book of Mormon that the whole face of the land was changed by the destructive events that are recorded in 3rd Nephi. How can researchers, at least LDS researchers who believe the record to be a true account (and hey, maybe I'm the one making assumptions), assume that landmarks referred to in the Book of Mormon prior to those events are even in existence today? Why are they looking for rivers and other physical features that may have been gone a very long time ago? Even without the natural disasters recorded in the book, the lay of the land changes over a long period of time. In a place we used to live in Maine, there was a tarn at the base of a small mountain that we passed daily. In the seven years that we lived there, the tarn went from being a pretty little pond to almost being able to mow it! Granted, it's a miniscule example, but I could hardly give a personal attestation to the effects of thousands of years. It makes my point--physical things change! (Had a look in the mirror lately?)
Also, it seems to me that the words spoken by the Lord and his prophets concerning these things are largely ignored in favor of the discoveries of men. Joseph Smith, Jr., who even the most diehard history "expert" would have to agree was the foremost authority on the Book of Mormon, seemed to have somewhat general ideas on where the Book of Mormon peoples were. Yet, once a person is dead, meanings of words and writings can be manipulated to support almost any theory. We do it to the Founders on a regular basis ("...the Founding Fathers did not intend..."or "...the Founding Fathers fully expected..."), and to the Bible as a matter of course.
All truth is important, but is it all equally important? And is it equally important at the same time and in the same situation? The Book of Mormon tells us that by the power of the Holy Ghost, we may know the truth of all things. I don't believe any of the researchers and authors will be able to "prove" their theories or fully substantiate their research until the purpose of the book is fulfilled--to testify to all the nations of the earth that Jesus is the Christ and through faith on his name we can be saved.