USA - S. Korea - Guangzhou - back to S. Korea

No, it's not a 144-hour available route. S. Korea - Guangzhou - USA will work.

No, you cannot do that because the route:

South Korea - Guangzhou - South Korea

Is not a transit, but a return trip, so it does not qualify for any kind of visa free transit scheme.

I advise you to insert some other country or eligible territory in your flight plan in order to be eligible for 144 hour visa free transit. In my opinion the easiest option would be to fly from South Korea to Guangzhou, then take a high speed train from Guangzhou to Hong Kong (West Kowloon station) and from Hong Kong back to South Korea. If you decide to do this, you should just know that you will have to buy your train ticket in advance and to take exactly that train you have bought in advance for your travel from Guangzhou to Hong Kong.