My pleasure. Yes, there are many grey areas, and when they are made truly "Black or White" is in a court of law. When sampling came out the US courts in particular were dealing with infringement cases of which were technically complex, demanded some knowledge of musical understanding, and, perhaps the most important, had no previous case law for the judges to rule from or to take guidance. Since then, there has been a history of case law built up, and therefore modern lawsuits can look back at precedent. But each case is unique - just like the musical art on trial - and therefore a ruling in one case cannot set a true metric or standard in another.
I just want to once and for all eradicate the notion that there is a paragraph in a dusty, leather-bound volume that states "If thou samples less than 2 measures of the original work they shan't be charged with copyright violation" This simply is NOT true. And where chord progressions, and even melodic lines are very nebulous in terms of their originality, sampling a recording without permission is completely black and white; it is against the law.