Blur and Displacement won't stack, as both grant some kind of concealment. Displacement has a special wording because it is not supposed to grant total concealment (usually the only way to get 50% miss chance), but I'm pretty sure apart from that it behaves like any other concealment does and won't stack.

Does blue and displacement stack?

You often see people cast blur and displacement because blur will last a long time and displacement gives you a higher temporary protection. People cast blur AND displacement so when displacement wears off, they at least still have blur going...

Does displacement and mirror image stack?

Yep. The subject's outline appears blurred, shifting and wavering.