The prefix lu- is a special inconsistency in the language. All natural languages have inconsistencies in them, but it is unusual to include inconsistencies in constructed languages. Since Dr. Okrand was constructing a language which was supposed to be like a natural language, he added some inconsistencies (some were added because of errors by the actors, which seems a very natural way to allow those inconsistencies to be there). When the subject and object are both third person, we usually use the null prefix (i.e. the absence of a prefix), but with one exception - if the subject is plural and the object is singular, then you have to use lu-. Since it breaks the normal pattern, it does make things a little more complex and difficult, but remember this not Vulcan, this is Klingon - it's not always going to be logical and it will present you with challenges.
Your sentence DIvI' pIch SuvwI'pu' can only mean, "The warriors blame the federations." Since the plural suffix -pu' tells us that the subject is plural, the lack of -lu' tells us that the object cannot be singular, and thus, even though DIvI' is not marked with a plural suffix, it must be plural. Similarly, the same sentence without any plural markers (DIvI' pIch SuvwI'), could be used for a variety of combinations of singularity/plurality: "The warriors blame the federations." "The warrior blames the Federation." "The warrior blames the federations." The one thing the sentence cannot mean (because it lacks the prefix lu-) is "The warriors blame the Federation."