Wyrrir

Yes, that phone call was the imaginary scenario.
I reality, the mother saw his son leaving room 701 very upset, checked the room, found the body and took care of everything on her own.

He didn't know it was in room 709.
Last night he left the dead body in room 701 and didn't know what happened with it after that.
The CEO actually didn't help him (that scene was an imaginary scenario), the killer just assumed he did, because he didn't know about his mother's role in the case.

Mostly, he focuses on his own survival, trying to prove his usefulness to the maffia boss. Because of that, he is looking for FL but not putting too much effort into it. So, no, he isn't annoying as an ex-boyfriend, not even significant regarding ML and FL's relationship.

Theory of the police, before the interrogations. They thought Yujin called the chairman, but that didn't happen. It was just an imagined scenario.
As I wrote, Yujin also thought chairman took care of the body (although, he didn't ask him for help), because he didn't know about his mother's role.

He didn't call him. That was a theory.

What crime did he commit?
Detectives assumed he had taken care of the dead body and the clues. And "Leo" thought the same. But actually he didn't even know what happened.

She regains her memory in episode 8/9. Her reaction regarding ML is positive, she understands his motivations. While she still has amnesia, there is a short break up (half an episode maybe) because of a misunderstanding.

At the end of ep 8.

Yes, she has a boyfriend. But in fact, he is just spying on her on behalf of the mayor.

But that scene (when Gu-hyeon is gathering his papers on the road and almost got hit) was in NGNL too (near the end of ep5).

No Gain No Love, for example.

He said in the end of ep. 10 he believed her. Then, they talked about it more in ep. 11.

She returned to her body in the hospital days/weeks/month(?) later and her clothes didn't change.
As for the cut on her back, before she traveled to the past she had fallen to the mirror crashing it. I guess people on the plane simply thought pieces of the mirror injured her.

Flex X Cop
Military Prosecutor Doberman
Behind Your Touch
Study Group
The Trauma Code
The Kidnapping Day

It's not an important part, just a subplot with a few scenes.

Script.

She is 35 (that's not almost 40), he is 24 (25 within about two weeks).
She couldn't be his mom.

Yes, there is romance, but it's not incest.

No, far from it.

I don't know. Maybe I see it too darkly.
But Ryong already was in a serious condition, and he was warned he'll lose his eyesight and ability to walk even if he returns immediately - but he didn't plan it at all.
And Byeol... scientists didn't have a solution (or even a slight idea) how bring her down to Earth. And if they manage to do it...? She spent her entire existense in zero gravity. What would gravitation do to her (or the forces during the landing)?
So, when I watched that scene I thought, it wasn't real, only an imagined moment from Ryong (like Eve's fantasy about their life together).