Are you sure that's how it works under Danish law? I many countries in these kinds of cases the burden of proof in on the defendant
Well no, the plaintiff has the burden of proof. Or in this case, since it's a criminal action, the state has the burden of proof.
One possible defense against defamation is to argue that the claim, while injurious, is true. That's an affirmative defense and shifts the burden of proof to the defendant, but that is not the only possible defense. Further, under Danish law, the truth of the statement is not a defense. My layman's reading of Danish criminal law indicates that a statement is defamatory if intended to be injurious, even if it is true. This differs markedly from the concept of tort in defamation that English Common Law prescribes. Under the law of torts, one is not entitled to a false reputation and cannot be injured by true statements. See what I've written above.
In this case, he would have to show that this persons actions alone were responsible for harm to his reputation. He would need to show his work is valid in order to do this (because this is the basis for the alleged slander).
Yes, if he argues that his reputation and esteem have been damaged, he would have to show facts that substantiate what his reputation is or was. Harrit wants to style himself as a legitimate scientist, albeit retired, who has undertaken (as a serious scientist) to question the official 9/11 story. However, Harrit's belief is not all that's relevant. There is no law against diminishing someone's reputation in his own estimation. The law is only against diminishing a reputation in the estimation of the public. As such he doesn't have the final word on what is reputation is. His reputation is how others perceive him, not how he perceives himself, and that is a finding the court will have to make independently, based on testimony from both sides.
Harrit's belief that he has not been refuted or rebutted is largely irrelevant. The scientific validity of his claims is largely irrelevant. He has chosen to embroil himself in a topic dominated by rhetoric and pseudo-science, and that is the public perception of who he is and what he does. Proving that his handling of the paint chips was sloppy and unscientific would not really help his defense because how the public
perceives his work is all that matters. If he were to turn out to be right, but the public universally believed him to be a crackpot up until that point, then that is the reputation he enjoyed at the time the remarks were made.
Forcing Harrit to submit his scientific findings to a less biased and more skilled practitioner for confirmation would be one way perhaps of calling Harrit's bluff. But it may not be a bluff -- i.e., Harrit may be willing to allow others to attempt to replicate his work even if he suspects it may fail. And it has so little legal value. I don't think the defense will follow a strategy of forcing Harrit to prove his science is valid. They only have to prove that Harrit's claim of a high reputation is really what the public has of him, not that of a crank. None of that really has to touch the matter of Harrit's actual work.