AGAA4
Thomas Cromwell played a large part in the downfall of Anne Boleyn. We can never know for sure how much influence he had on Henry.
You are right - there is much of the story that is supposition rather than fact, which is another reason why I think there is still so much interest in it after 500 years.
It is debatable whether Cromwell engineered Anne's downfall at Henry's behest - in order to get rid of her because she hadn't given him a son, and he had replaced her in his affections with Jane Seymour - or whether he did it for his own reasons.
It has been suggested that he took against Anne as revenge for the execution of his mentor, Cardinal Wolsey, which is a possibility. As you say, though, we'll never know.