Overland travel from South Korea to China won't be easy... you need to go through North Korea, which I assume, without being 100% sure, doesn't give transit visa easily...
Also, overland from China to Thailand is possible, but a long journey, the fastest way is crossing border into Laos. Take a bus from Jinghong Yunnan to Mengla, then take the bus to Lunag Namtha. The Laos Visa can be bougth at the border for $30 (double check this) +2 photos. Another option is going through Vietnam, but you'll need to also cross Cambodia or Laos.
I can't help you for the rest of your questions, sorry...