Need to maintain gelato at 10-18 °F without electricity I will be vending a soft serve/gelato at events and festivals this summer. I need to maintain its soft frozen temperature between 10 and 18 °F (-12 to -7 °C), but most events are very limited on power. I would like to use a cooling bath that needs to be safe around food.

32° is easy (ice) and 0° is easy (ice and salt). Should I just add water to the ice and salt mixture creating a slurry that would be a less efficient system? How difficult would it be to maintain a semi stable temperature? Are there other cooling bath mixtures you would recommend? I've done some research and found many of the alternatives are not food safe.
You could experiment with salt to ice ratio. Somewhere around 15% salt to ice ratio you get freezing in 10-18° F range. 

Once you are at 10-18° F, you should have extremely good insulation to keep the temperature stable, have a thermometer placed very close to your gelato and constantly check that, have lots of frozen ice+salt mixture and minimized the exposure of your frozen content to ambient environment.