I might have missed something here.

#2 he was totally wasted

So completely totally wasted that he came on to Kang Jinha , even though he thinks of himself as straight, which completely got jinha's hopes up and then crushed when Siwon forgot.

right...thanks.

2. He was drunk, mental lagoons are common after that state.
Also, you usually remember stuff that is important.to.you or something that took your interest somehow. It depends on the impact, how.does affect you. And sometimes due to repetition (abilities, songs, studies).
And I think it depends of people too, their memory and how they work with it. For example I remember some things that I did when I was 1, 2 and 3 yo, but I can't remember what clothes I wear regularly (maybe if there was an event I can remember), also, there have been times in that names and faces are hard to remember, but there was this girl that I met when I was 4 (school) and I saw her later when I was 20 and she remembered me! We hadn't seen each other for 16 years and we weren't even friends(I'm not sure if we even talked) but she still remembered me!
It was incredible for me... moreover when there are so many things that changed due to growing up: hair color, body, face features.
Edit: oh, wait, maybe she remembered me cuz I peed in class because the teacher didn't let me to go to the bathroom. ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄

3. I think he is socially awkward because he was isolated by his mom.
Also he was abused and that treatment might have left him with traumas and difficulties to socialize (there are persons who don't have a traumatic experience and still they have trust issues or they have a personality that isn't compatible with a lot of people).

We don't know about their full stories and interactions, so I prefer not to judge them. Of course I don't like some of their actions, but I think they are comprehensible.

Thank you for the long reply. It's like you said, cos I also have this memory blackout experience after heavy drinking. I'd forgotten how I got home after that, but a couple of days later, it eventually came back. But I think some people have more alcohol tolerance than others.

For Jinha, I din't mean the past traumas, that was understandable for him to be a shut-in. It's more like his attitude towards Siwon -- he's like hot and cold. (And his behaviors become more unpredictable in the next chapters too, hehe.)

#1 probably just a regular beautiful stone, the feelings jinha have turned his wish to come true (Gods magic like Kimi No Na Wa movie) and the stone is the medium (He wish to see siwon even at least in dream)
#2 Hes drunk and totally wasted, when kang jinha talk to him outside, kang jinha probably confess that he is kang jun from the childhood days in which siwon remember and led to intimate scenes. Kang Jun in chapter 15 is Kang Jinha (maybe his father save him or his mother remarried thus changing names), Si won remembers his dear friend in the name of Kang Jun but i guess he didnt much remember his looks since Kang jinha always wear long sleeves and face mask.

This is what probably happens in retreat--- They have met in retreat, Kang jun-now Kang jinha remembers him but siwon didnt remember his face and his name changes, Kang Jinha admits that he's siwon's precious friend from before, siwon acknowledge it. But he forgot it. That drunk moment of them is the only time kang jinha admit his identity but siwon forgot it, its like going through their school life and siwon didnt remember it, so kang jinha thinks siwon avoided him all those 3 yrs after freshmen days.

#3 Well siwons personality is.. He only know siwon as his friend, he probably falls for him when he sees him cry in ch. 15 in their childhood days. Being the only friend, and such a great kind friend, Kang jinha treasures him and put him as the most important person in the world. Kang jinha's rare,. After their separation, he continuously thinks of one person (none existant in reality, please give me some Kang Jinha) and some scene probably happens like the situation got so high that Kang Jinha almost died (maybe by his mother's hand) he seems hopeless but he put Siwon as the reason to keep the will to live. And now after the retreat, Kang Jinha thinks Siwon avoided him afterall that loving scenes so he turned to some bitter prick who hates the persons that couldnt keep their promise (hes referring to Siwons promise to stay by his side when theyre young, bitter reminiscenes) and he hates the one whos irresponsible (after the drunk retreat, He thinks Siwon treated the thing about them as nonexistant)

And now he turned possessive and kinda psycho inside. Why? Because that thing drags on for 3 yrs., if time heals, it also worsen things.

Such a good insight! Yeah, I guess Jinha must have said something at the camp too, but since Siwon forgot, he might think what's the point?

I don't know whether this actually reflects the mangaka's intentions at all, but something called attachment disorder happens a lot in people who were abused as kids. It can happen when you don't have a good/stable bond with your primary care-giver as a child or when you're a baby (even if they just like you sometimes but ignore/dislike you at other times). It can seriously affect your ability to properly connect with and trust other people all through adulthood. Google it, it's real!

Thanks! ;)