If you get an H2 visa in China, you must study in Korea for three days before you can apply for a foreigner's landing permit. If you have a C3 visa, you should study in Korea for 6 weeks first, then exchange your graduation certificate for an H2 visa in Korea, then study for 3 days, and then apply for a foreigner landing visa.
It won't be invalidated. You can go to Korea anytime and anywhere within 3 years from the date of obtaining the H2 visa without going to Korea first.
There is no employment restriction for visiting employment, and you can work freely.
Then you can go to Korea at any time in 3 or 3 years, without review, 1 year without reporting once.