Bruv, we're still in episode 3? All of the comments you gave are easily answered if you actually watched the series and tried to think about what's currently happening and not what's spoonfed to you.

1. Literally answered in Episode 3 wherein she mentioned that he let Peach stay because he did come all the way to Korea from Thailand and showed her proof of how close they were. Also, you can clearly see she was weirded out in the beginning when they met and very suspicious because Yoon Oh was dead and now here comes a stranger telling her that he was there to meet her dead son. If you actually paid attention to her facial expressions throughout episode 1, you would have not been leaving this stupid argument. 

          2-3. She didn't tell 'them' to be quiet, she told Peach to be quiet as she was just bothered by the noise. And in a big house, practically a mansion, where it's even located in a secluded area, he would've been incredibly loud which bothered the mom. Furthermore,  she didn't even reply to Yoon Oh telling her that she always tells him to keep quiet. In addition to that, just please STARE at her facial expressions which are good enough to explain a lot. The same type of response for your third point. He wasn't in a position that involves two people that would be thought of as mentally ill.

          4. She never scolded Yoon Oh, the conversation was about how Peach became friends with Yoon Oh and she mentions that they have such opposite personalities. The topic also became dramatic when she found out that Yoon Oh liked Thai food and ONCE AGAIN, look at her face when she realizes that she really doesn't know a lot about her own son. Afterwards came the opposite personalities topic where she talks about the bad qualities that Yoon Oh had in contrast to Peach. The director deliberately made it seem like they were responding to each other since the viewer didn't know yet at this time that Yoon Oh is dead. Obviously, if they didn't show the mom "scolding Yoon Oh", we, as the watchers, would have already had an idea that Yoon Oh is dead or there's something wrong. But if you actually paid attention, which I see that you didn't, she wasn't directly responding to Yoon Oh at all.

The Storyline isn't messed up, it's literally slowly being uncovered as the episodes go by. Do you just want everything to be obvious and spoonfed to you on the go? The chopsticks and the people at the restaurant not seeing things flying due to a ghost holding them is sooo stupid. Literally many shows have done this style of having a conversation with a ghost with other people around them and now you're having a problem with this particular series doing the same thing. Everyone is weirded out by Peach at the restaurant because he was talking and shouting by himself but is also considered rather normal as he is drinking. 

Lastly, "mind disturbing plot" is a compliment, is it not? That is literally the intention of the series? 

If you're going to be such a hater that you would even copy-paste this hate post on a different post with a totally unrelated topic, at least be intelligent about it. Here's the attention that you seem to be desperate for. 

Well in point 4 I was talking about the part where Peach yells at Yoon Oh, and his mom doesn't find it weird to see Peach talking in air or with air for that sake XD

Nice escape from the topic.

I intend to trap your ignorance into it and liberate your little self

You are already trapped! You just don't know that yet

Whatever,  I still stand correct - It is indeed the most horrible drama ever.

Yep. You are just blinded by Yoon Oh's cuteness