The meat will not accept cure below34-35F... Keep the temp around 38 and you'll be good to go... I'd raise the temp and add 5-7 days....
Also, when your brine freezes, it does not include salt in the ice... only water... Ice bergs in the ocean are water only.. the salt precipitates out of the brine... The ice "may not" include sugar and cure also... I don't know if it precipitates out, but a SWAG is, it does separate...
According to GOOGLE: "Plain water freezes at 32 degrees F, but when sugar, or salt, or other solutes are dissolved in it, the freezing point gets lower. This link says the standard 4:1 solution starts freezing at 26-27 degrees F." which is around - 3 C.
When you add sugar to water, the water (the solvent) becomes a solution (a solute dissolved in a solvent). Adding sugar disrupts the liquid state because sugar molecules move around aimlessly, making the liquid water molecules less organized. Sugar molecules don't pack together with water molecules, so when the water molecules start to freeze, the sugar molecules remain in the liquid water. When the water molecules create ice, the sugar molecules have a smaller volume of liquid in which to move.
What happens when you freeze salt water?
The reason has to do with the behavior of ocean water when it freezes. When salt water freezes, much of the salt is expelled from the ice or gets trapped in pockets of salty liquid water within the ice. Eventually, most of the salt makes its way into the water just under the ice.
Can you freeze salt out of water?
If the ice forms very quickly, however, the salt can get trapped in pockets in the ice. You probably know that salt water freezes at lower temperature than pure water. ... But if you freeze it too slowly, the water outside the cells forms freshwater crystals, leaving the unfrozen water with a lot of salt in it.