Even if the surface temperature of the solar collector focus point is 1500C, the molten salt flowing on the other side of that wall must be at a significantly l

Even if the surface temperature of the solar collector focus point is 1500C, the molten salt flowing on the other side of that wall must be at a significantly lower temperature in order to transfer heat quickly enough from the small area. Then you will have a thermal profile in the wall, say from 1500 to 1200 C, and furthermore a thermal boundary layer in the fluid, say from 1200 to 800 C.