Seoul to Shanghai(2-hour layover) to Hong Kong

You don't need a visa, you'll get that 24 hour transit where you can collect luggage and check in to your next destination. Naturally it's only 2 hours and you won't be exiting the airport so you will be fine without a visa for shanghai. Also when you check in for your flight, just confirm with the ticket agent if you definitely will need to collect your bags in Shanghai , or they will be checked through to the final destination.