A Underworld Trilogy plot hole? *answer if you've seen all 3*?

You make a good point. I don't think you missed anything. Maybe Viktor had Selene's father make William's casket fit the key (necklace) he had. But then towards the end of the movie Lucian took the necklace and ran off. So I don't know how he got it to fit the lock. I'm not entirely sure, but we can only ume. No matter what they were all good movies.