Cassandra and Eve are locked in a walk-in freezer. Cassandra sets the thermostat to try and freeze the metal of the door so that they can shatter it. This is impossible for several reasons. First, the cooler system would never be able to go that cold, second, if it did get that cold they would freeze to death long before the metal got cold enough to shatter.
The onions in the freezer were not frozen.
The warning sign on the secure door to the barn reads CYNDI SAYS KEEP OUT, but the character's name is spelled Cindy throughout the episode and in the title. However, later in the episode, Ezekiel notes that she deliberately changed the spelling as part of her establishing a new identity.
Agnes has no reason to reveal there even is a secret ingredient to Baird and Cassandra, let alone what it is.