When Gold talks about magic never being able to be created or destroyed, only changing forms, this is a direct reference to energy. The first law of thermodynamics states that "energy can never be created or destroyed, it can only change forms."
This episode has another appearance from Marvel comics; this time it's Wolverine.
Will, picking the library door, says it is a 6 pin lock, "Not the easiest." Granted, the more pins the harder to pick and 5 pin locks are still the most common, by far. But, far more important is the shape of the pins and how tightly machined the parts are... the less wobble, the harder to pick. But irregularly shaped pins (spool, serrated, mushroom) are the worst.
When Elsa and Anna are hiding from Ingrid they are in a room in the palace with portraits of their relatives. One of the portraits was King Edward VII of England whose wife Queen Alexandra was Danish just like Hans Christian Anderson who wrote the Snow Queen.
Elsa, preparing the table for dinner with Anna, mentions "julekake." That is a traditional Norwegian Christmas yeast bread made with Cardamom and candied fruit. It is in the shape of a boule (round loaf) or a braided wreath.