During the 3 days of dark (totally personal preference), you should be past your flush period, and anyway the soil won't dry out if the plants are in the dark.

Since you posted in hydroponics, I assume you are in hydro medium. In which case they still will be on the res that has your flush solution- anything from plain pH'd water to a clearing solution.

Tips curling up sounds more like acute heat stress/burning than K def.

Choosing your flush solution depends on your goal. Within one week of your chop date, I would not give any food. If a flush is mid-grow, indeed you should use a 1/4 strength nute solution as the last part of yoru flush.