I THINK jsng confessed first. i explain: in my head, mhn is a Yearner who'd have ended himself first rather than make jsng uncomfortable. in MY view of him, mhn's someone who gives more than he takes (he takes care of others, he keeps things bottled up inside, and he maybe lets two people at most see him vulnerable). but he's sincere, so the love and affection he has for jsng is evident in each of his actions.
jsng is aaaaaaaaa a mess. not anymore, but he was. the kind of nervous mess that overthink anything and spends hours analyzing was he thinks and what the others are thinking too. and I think that's why jsng confessed first. a hesitant confession that surprised them both because maybe they were doing something perfectly random like... folding socks or something. a tiny, stammering "hyung, i'm in love with you." and BOOM.