The 6m figure was known and established long before a single author writing a book in the 70s, in fact it was documented during the Nuremberg trials using Nazi documents and records, later refined by comparing pre and post war census numbers. No idea why you think the figure came about 25+ years later.

Around 12m (mostly from 6m Jews, 3.3m Soviet PoW, 1.8m Poles) died. Putting people in camps with insufficient clean water meaning they died of dysentey doesn't somehow mean they can't be counted - why were they not able to access clean water? Because the Nazis created the camps without it. That argument doesn't work as they wouldn't have died if they weren't rounded up and put in the camps