My testimony of the Book of Mormon does not depend on scholars being incorrect about the authorship of Isaiah. If it turns out that Isaiah 48-54 wasn’t actually on the gold plates and Joseph felt inspired to add that material to the text of the Book of Mormon, I’m fine with that.
This kind of an approach may not sit well with some Mormons, because we tend to assume that Joseph had very little influence on the textual form that the Book of Mormon took. But is this assumption necessary?