One thing that I would like to see is for them to detect the planet using the techniques that allowed the deduction of the existence of the other planets. It would act as a validation of the non visual technques. This would make the position of the naysayers much weaker.
The naysayers do have the right to disagree. But to date I have not seen any of them bring forward an alternate explanation of the data. They only say that it doesn't prove the existance of the deduced planet. In disagreeing with a theory an alternate explanation for the data should be provided or a serious flaw in the data or deductive method used should be shown.