Is ice always at 0ºC? Can it be colder? Can it be warmer? What about an ice – water mixture?

No, ice is not always at 0̊ C.
Yes, it can be colder.
No, it cannot be warmer.
The temperature of the ice will remain constant until all the ice is melted to water. This means that the temperature of the ice-water mixture will remain at . So, the ice will neither be warmer nor colder as the temperature remains constant.