Certified GKH Gardening Expert
I am afraid that you are right outside of their habitable zone. A cooler than normal year may damage the plant pretty badly.
They can tolerate pruning, both root and top. Just be sure to do this when the weather is mild, and not at either extreme.
Here is an article that will help with the plant's care through the cooler months: https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm
Here is an article that will help you in the case that they do get a little frostbite: https://www.gardeningknowhow.com/houseplants/crown-of-thorns/frozen-crown-of-thorns-plant.htm