well they made it look like he ran away on his own so i guess it makes sense he would suspect him. but I don't agree with his silent treatment towards him :(
He didn't know, after he came back Nakyum was there asking to go home. Also Jihwa didn't call Seungho in order to distract him, I honestly think you'll are overrating too much the author. The kidnapping plan was just to create the unpteenth misunderstanding between Seungho and Nakyum and a side couple.
I feel like by this time there ought to be some character growth though but guess not haha
i mean....didn't seungho basically busted jiwhas plan or something? he went to meet the guy and was all, dont fucking mess with me you lil bitch, or smth. he knows nakyum was in danger right? then suddenly nakyum disappears. like....two + two = four.....it should've clicked that something was wrong.
or....did he not know? soz I dont remember, i have short term memory and not bothered enough to reread