I have to disagree with a couple of points you make. First, there are nine stories, not eight. This may seem like a quibble, but the the book is called, after all, "Nine Lives." If you missed that you have wonder what else you may have missed.
Well, I'll tell you one equally big thing: You say "in place of an author's voice you get a very stripped narrative...." While it is true the stories (except Wells') were not told in the first person, they are clearly told from the perspective of each main character. The grammar and syntax ("voice") of each is distinctly different, and the author tells each narrative just as he must have been told it by that character and a few supporting characters. This is why all the other stories are not written in the first person-- because supplemental information was gleaned elsewhere, just as the author explains in the "About this Book" section. Also, to have nine first person stories would give the book an entirely different-- to me less effective-- style.
So making that distinction gives one a completely different reaction to Baum's narrative choice: Baum lets each character, clearly oblivious to how it must sound, tell his own story. Baum lets the characters tell what they did and why they did it and offers no judgments-- the actions and thoughts of the characters "trumpet" for themselves. Hence, you hear the voice of the coroner tell stories about himself that show what a complete and utter idiot the man was. Another character sounds perfectly reasonable when he talks about administering corporal punishment to his students with a PINE BOARD. These characters are not black or white, bad or good. As a New Orleanian and a discriminating reader, I was relieved that the author didn't hit me over the head with his own opinions-- it's all right there for the reader to discern, right out of the horse's mouth. Now THAT'S magic. This book is fascinating! I beg to differ with you-- the color is all there--jaw-dropping Technicolor, no less. Maybe the author's style is too subtle for some. For that reason, this would make a good book club selection.