He's desperate. Yes, he's being rude and offensive (the joke will be on him, cuz I assume he'll be Jihan's lover) but Jinha hasn't been the nicest person either. Jinha also misunderstood a lot of stuff, he's mad because his first love didn't remembered him but he changed his name and always wore a mask when they were kids.
It's obvious he knew Siwon didn't recognized him, so he needed to make it clear, but he decided to do it when Siwon's was drunk!
Jinha can react like that too, of course it would hurt you if that important person didn't remember you and your promise, but he has to confront the issue and take responsibility for what he has done too. He doesn't know what's going on with that gem, but he knows he did something and he knows there are risks: what happens in the dream, happens in reality. He is aware, cuz he knew that if Siwon fell he would surely had died.
But his pride hasn't let him do the right thing, he's too hung up with the "you didn't remember me,I hate you!", that he obviate the other important issue: the freaking dreams becoming truth.
SSiwon didn't do much when Jinha mistreated him (work group), yeah he didn't do the best work, but Jinha attitude wasn't the right, he was judging Siwon without even giving him a chance to do something. He only got awful when he figured that Jinha did know something about the dream and that he didn't want to tell him the truth.
I'm not saying Siwon isn't being mean, he is leaving all the responsibility in Jinha, that's awful. Although, I'm not really sure what did happen in the retreat, if it's just what we saw, Jinha did take advantage of Siwon because you shouldn't do that to someone who is drunk, but as this is fiction, it's allowed, it's just part of the plot, I don't know,"Siwon finding his true feelings", that kind of thing. ╮(￣▽￣)╭
So, Siwon is being mean but Jinha has been mean too, they need to talk quietly without so much prejudices, however that's not how people work.(⊙…⊙ )
I'm thinking of the possibility that Jinha told him he was Jun and Siwon accepted his feelings and then kisses time ( ° ʖ °)
wai have to be high note? wai's siwon always talking w high notes.. ;;; it's makes me sad..
if am in jinha's position..... i'll do the same wat he do lol