What We Sang experienced was rape. I can't believe they're treating it like a joke. He does not have to get married to her, he's a victim! Hwa Sang even tells her she committed a crime. This drama is crazy, but this particular situation is next level.
For Young Bae to have taken over Ki San's spot, he needed to look like Ki San. My first guess was that Young Bae switched bodies with Ki San, since Ki San's body was never photographed after the police officer found Hwa Sa and him in the car. My problem with this theory, though, is that Hwa Sa (in Hae Yi's body) recognised Young Bae immediately on TV and in real life. What is the truth lol.