Tom, if I might add this postscript. I'm afraid I can only speak in more generalized terms and in the broader scope; for some of the specifics to your questions I'm afraid we will have to rely on those that know more than I, or perhaps from folks in those regions in question. For subjects as specific as Plattdeutsch (and high versus low German), there has been a wealth of information already shared on this list, perhaps before you entered the scene. Here you could simply follow the instructions sent to you earlier today by administrator Juergen and search the list archives for any key terms of interest (like "Plattdeutsch", etc.). You will pull up quite a bounty even before your fingers have stopped. The rest we will have to rely on others to contribute if they can, or correct my faux paus' (paws in the American <gr>).
Of course when all else fails (including the ubiquitous and all-knowing Internet), don't overlook any decent sized library. The library remains to this day the greatest repository of freely dispensed knowledge to be found, with equally capable folks manning the reference desks in just about all. Even with the WWW closing in on it's heels!
Jb