::The "He failed to completely understand the book" example is not a very good one; "He failed to understand the book completely" is not at all ambiguous; it can only mean that the subject's understanding was incomplete - when the adverb occurs at the end of the sentence it really isn't possible to consider it as a modifier of the word "failed" alone unless the sentence is spoken out loud with unnaturally strong emphasis being placed on the word "completely
(a) Not true; both interpretations for "failed to understand .... completely" are available with broad focus intonation
(b) Regardless of the effect of intonation, intonation is not available in written language, so the problem remains in that situation.