The approach of the authors, as expressed in the preface and elsewhere, was not to argue with past conclusions about the massacre, but rather, to take a fresh look at the research and go where it took them. That is why your supposition about this being an "apologetic" work is so far off the mark.
Since the authors did not set out to debate anybody, or to refute anyone's theory about Brigham Young being the instigator, it is a natural course that they would deal with the events roughly in chronological order. Since this has evolved into a two-volume project, that logically leaves the cairn incident to volume 2, which is devoted to discussion of events following the massacre.
What does this have to do with whether or not the authors addressed BY's attitude towards the massacre?
Incidentally, I don't concede the accuracy of your rendition above of the alleged "coverup and and lack of church censorship [sic] of the known murderers." But I too eagerly await volume 2 and expect it will be enlightening. Of course, you have the reading of volume 1 before you, which you have yet to do despite having plenty to say on the subject.
I have said nothing about the content of the book, other than to ask questions of those who have read the book.
So, since there is no volume 2 to read yet, we can discuss the cover-up and why you apparently do not believe it occurred. Do you believe BY was not able to immediately discover the names of those involved, were he to desire those names?