Both premiums stack. Working extended hours after 22:00 earns 1× (base) + 0.5× (extended) + 0.5× (night) = 2× the regular rate. Enter those hours in both the Extended and Night fields.
(40 hrs/week + 8 hrs weekly holiday pay) × 4.345 weeks/month ≈ 209 hours. This is the standard for a 5-day, 40-hour workweek in Korea.
Yes. Workplaces with fewer than 5 regular employees (상시 근로자) are exempt from mandatory extended, night, and holiday premiums under the Korean Labor Standards Act. However, minimum wage law still applies.