Sangwoo is treating Bum nicely because he's prolly noticed the house has been bugged.
He's pretending to be a nice person because he knows they're being listened to. The sex in ch. 35 happened outside of the house and it's a possibility Sangwoo did it outside because he was suspecting that the house was bugged. What they said once they got inside was mainly Bum just telling Sangwoo he likes him. The following night Sangwoo probably knew Bum is too dependent on him to say no so he went for it to give off the impression that they're an actual couple. The following morning Bum said out loud that he liked it, giving off the impression that it was consensual.
Yeah, to the detective it probably came off that way because he knew the truth but it couldn't be used against Sangwoo in court since Bum said the following morning he liked it and even if the detective reported rape or assault, it wouldn't hold in court because they would need Bum to witness that was the case and he's too fucked in the head to betray Sangwoo now that he has the false impression that Sangwoo actually cares about him. Sangwoo is probably not trying to convince the detective that he's innocent, he's just careful to make sure he doesn't say anything that can be used to prosecute him if/when the detective goes to the police with all the material he's recorded.
Forgive me father, for I have sinned.
That's actually a common misconception. 13 is the age of consent set by the federal law as a base line that prefectures are not allowed to go under. Then each prefecture sets their own age of consent, most of which are currently 18. Similarly to how individual states in the US have their own age of consent. The federal age of consent in Japan is severely outdated because all individual prefectures have a much higher age of consent in their local laws. But if a prefecture decided to lower it to the federal standard, my guess is that the federal age of consent would be raised pretty effing soon. As for now, it's just not necessary.
Okay, rant over, I just roll my eyes every time I see someone spreading that misinformation without even bothering with a simple google search.