ad Fingerhut:
(…) think it’s difficult to speak of ‘usage in Germany’ as there are huge differences according to the dialects (…)
You are right, this is why I wrote “by some Germans”.
(…) I wouldn’t consider ‘wrong’ usage of tense etc. as a mistake, though. (…)
I guess, here we’ll just have to agree to disagree 
To me wrong usage of a tense certainly qualifies as a mistake and if anybody studying German as a foreign language would use the pluperfect the way some of my German relatives do, this usage would undoubtedly be marked as wrong at any exam those learners may wish to take.
There are certain differences between dialects and between “slang” and “standard usage” of a language and there are certain regional grammatical pecularities. For example, the different usage of articles in Austria as compared to Germany (der Keks - das Keks, der Joghurt - das Joghurt, die Cola - das Cola) or different usage of auxiliary verbs (“ich habe gestanden” vs “ich bin gestanden”). All these differences, however, will be dealt with in a standard grammar book or textbook for learners of a language.
I have never ever found any grammar or other textbook where the usage of the “Vorvergangenheit” I was referring to in the examples I gave was taught or accepted as correct.
Of course, you will hear lots of people who say and/or write sentences like “He don’t know nothing” in English for examle. This usage may actually follow certain rules in some linguistic “subsystem” (I don’t mean that in any derogatory way, I’m just not a linguist) but it certainly is not correct if you take “standard English grammar” as a basis. I am very much aware of the fact that language and its usage is not as homogenous as it is sometimes portrayed but in my opinion it is not a totally vague system either.
If there were no rules, how would we ever be able to study a foreign language. Patterns of usage are also some sort of rules.
(…) It’s not standard, it’s dialect or slang, but it’s not ‘wrong’, if everyone uses it like this in a certain area. (…)
Again, I’m afraid I’ll have to disagree 
“Ich habe ihr gesehen” is widely used where I live but it certainly is not correct “standard German”. Try and tell any child to write that sentence at school and see what grade he or she gets 
It may follow a certain pattern in the local dialect and within that specific subsystem it may be correct, but I think that SolYViento was referring to what we may call “standard language”. I don’t think he was referring to the myriad of diverging forms that exist in local variants of a language.
I know there have been many discussions on what should be considered to be “standard” and what not. Nevertheless there is agreement on a large set of rules (at least in the languages I have studied).
The other examples you gave refer to different pronunciations and that to me is indeed a totally different matter.
(…) A mistake would only be something that only you yourself use differently to everyone else, in my opinion. (…)
So, if I find just one more guy using the same grammatical form as I do, would that make it less of a mistake? I don’t think so. But, of course, that is just my personal point of view.
To sum it up, my understanding was that SolYViento had some sort of “standard version” of language in mind (mostly that’s the one we are taught at school) and was wondering if there were native speakers who consciously or unconsciously deviate from that standard form.