Someone who accuses another of committing "a very serious error of thought" should probably not compare academic misconduct to murder or rape. When was the last time anyone went to jail for academic misconduct?
Does Piketty's book disclose that he altered the numbers he worked from, such that they do not reflect the sources he cited? If not, that would constitute serious academic misconduct under some (relevant) standards.
Of course, he admits to altering his data after someone else noticed it, but some of his changes are apparently plain errors, and the rest are not annotated or explained. They also push the data in a direction that Piketty finds useful (long before the FT article came out, scholars criticized the book for making overblown conclusions that were not well-supported by the data it presents), which is further reason to be skeptical about the nature of the changes. Data manipulation doesn't have to be intentionally biased in order to be consistently biased.
To pick just one flaw in your new link, I cannot reproduce the 6% discrepancy that Reed claims in his The Guardian piece between data sources (b) and (c) -- I calculate it as an average 3.8% difference in the top 10%'s wealth over the eight-year overlap, dropping to 3.3% if you exclude 1974's anomalously large discrepancy, and in either case dropping a bit if you round (b) to whole percentages to match (c). On top of that, the difference goes in the other direction for the top 1% estimates, which makes suspect claims of systematic bias between the two, rather than sampling noise. I think I'll opt out of being "highly inform[ed]" by bad arithmetic, thank you very much.