It was another Yakra that was pursuing the charges, no? I assume the Yakra line knew of it, while most did not. Yakra then proceeds with his plan to overthrow the King and start the accusations and the trial. Since barely anyone knows of this treasure, they can easily assume that the King is indeed stealing, especially if there is a convincing argument. It is not like the jury has the "CSI effect", where one needs indisputable, forensic evidence/proof that the King, did indeed steal a rainbow shell. They won't be dusting for fingerprints, scanning for fibres, hairs, other sources of DNA, etc. They proceed with the trial as a trial used to be done, with convincing arguments, non-DNA evidence and witnesses.