(this is for whoevery wants to do internal RTC wakeup with out external interrupt)
So i have read the whole thread and tried everything, and i can tell that these 2 functions are totally useless and they probably are the default power mode for the chip , and the wait function is the same as using delay(***) whatever number , so if you need a program to wake up internally every couple of seconds , just use delay() in loop and You are all set.
sd_power_mode_set(NRF_POWER_MODE_LOWPWR);
sd_app_evt_wait(); // puts the nrf52 to sleep when there is nothing to do
Small note: the turning off external flash code saves around 18micro amps 