In 1884 the Society for Psychical Research sent Richard Hodgson to India to investigate the phenomena at the Theosophical headquarters at Adyar, above all the Mahatma Letters, messages from the Masters that materialised in a shrine cabinet. Hodgson reported in 1885 that the letters were written by Blavatsky herself, that the cabinet was rigged, and concluded with the sentence that followed her for a century: that she was one of the most accomplished, ingenious and interesting impostors in history.
A century later the same Society published a critique of that report. Vernon Harrison, a professional examiner of forged documents, published J'Accuse through the SPR in 1986 and in expanded book form in 1997, finding Hodgson's handwriting analysis amateurish, his procedure prosecutorial rather than investigative, and his report unreliable as evidence.
The distinction here is the whole point, and it is routinely blurred by writers on both sides. Harrison impeached the prosecution; he did not establish the defence. Showing that a 1885 investigation was methodologically bad is not showing that the letters were genuine, and Harrison did not claim it was. Theosophical publications sometimes present the 1986 paper as an SPR retraction and a vindication; it was neither, a learned society publishing a critic in its own journal is doing what such societies do. What can honestly be said is that the case against her was made badly, and that the case for her was never made at all.
None of this settles what she was. Olav Hammer's analysis is useful here because it sidesteps the question: whatever the source of the teaching, the strategies by which Theosophy claimed authority, appeal to hidden Masters, to a lost original wisdom, to Eastern antiquity, were themselves a nineteenth-century construction, and they were enormously effective. Her influence on everything downstream of her is not in doubt by anyone.