72 hours Shanghai

Hi !
we are 2 guys (1 french and 1 spanish). We are currently participating on an internsip in south Korea, we both have 6 months visa for this country and we would like to visit Shanghai for the weekend. However, we dont know if we can take profit of the 72 hour free transit if we are coming and departing from the same country (South Korea).

In other words : South Korea - Shanghai under 72 hours - South Korea. Is it possible?

IF NOT, we would like to make a round trip for a few more days.

South Korea - Shanghai under 72 hours - Tokyo - South Korea. would this itinerary be posible?