Rainus: I don’t have time to debunk each tangential point you raise, but I will make my main arguments.
1) You have not mentioned a single name of a Rishon born in Germany before Rashi and the Baalei Tosafos, who were in France, to support your assumption that the community and Yeshivos in Germany preceded them.
2) Even if the community in Germany did predate the other European communities, there is no proof or logical reason to assume that Frankfort or any of the other German cities preserved the original minhagim more accurately then the communities that migrated to Central and Eastern Europe. Why should we assume everyone who moved to Poland and Lithuania wantonly chucked all their traditions overboard even before Chassidus and all those who remained in Germany faithfully preserved the “authentic” traditions without deviating by a hairsbreadth? And we see that each city in Germany had it’s own minhagim, how do you know if Frankfort, Hamburg etc is the “true” tradition?
3) I proved by the way of examples (3 hours and talis, and Kabbalas Shabbos can be added to the list) that the German communities did NOT preserve their traditions completely unchanged (regardless if 3 hours is a chumra or kula, it is certainly a change). If we see they made these obvious changes it is only logical to assume they also made numerous more subtle deviations. It is much more logical and reasonable to assume that in EVERY community minor discrepancies crept in over the centuries, all of them are valid but none are identical to what was practiced 1000, and certainly not 2000, years ago.
4) Why would the Magen Avraham, who is probably the foremost authoritative sefer on Shulchan Aruch Orach Chaim, have written all the things he did just for the fun of it?