The 4-piece bonus effect does not stack with itself. The 4-piece set bonus effect is not activated by Swirls on self, such as from the field of Dandelion Breeze.

Does Wanderer's troupe and instructor stack?

Wanderer's Troupe has the same 2pc bonus as the instructor's. Hence, stacking these will make Kazuha unstoppable in reaction damage.

Does elemental mastery stack?

The Elemental Mastery-increasing effects of this Constellation do not stack.