The coffin didn't have a latch, nor any hinges or other means of sealing it. If he had to guess, the pane of glass had probably been melded onto the stone with Magic - another spell that had been lost to time. If he chose to he could push the cover away and get access to the corpse and the necklace. The Ghost didn't seem to have any issue with the Moros' that searched for ways to disturb her remains.
At least she didn't react to it for now.
No roll needed to push it open or break it directly.
Seems like we're nearing the finish line!