The only information I have about the spine width is exactly as you put it, the illustration provided by the template, and which changes based on the number of pages. LR4 does allow you to input page numbers and paper type, so it is my assumption that they include that in the calculations for "how wide the spine will be and therefore, the illustration or box allowed for the spine." The other information is the test book I had made with exactly those same variables. And I do believe that it may be too thin to accomodate legible text. A real dilemma, as I want to make this series which will include dozens of volumes, but I also want to keep the cost to a minimum, and I also like the soft cover aesthetic for this particular series.
Anyway, let's see how they do with the "spine image" once it's all done. I will let you know.
Thank you for al your input.