missing scenes or bad translation?

I'm vietnamese and I had unanswered questions too.
Although, I think I have answers to your questions:
>>why did he have to stay in that room?
Seemed like he was depeneant on work from his gang friends and was their meeting place and probably a place to crash during the day and stay out of public view
>>why did they make him drink petrol?
At that scene, they used that to initiate him as much as threaten him that they'd kill him if he turned on them

I didn't really understand the relationship between the guys that stole his bike and the landlady. I did not understand the whole thing with the landlady's son. Was he ill or stunted in some way? It didn't come across as relevant, unless it was to make the symbolic comparison with the fish whose lives were always on a string (his mom always called him "my little fish" and he made fish noises). And for the sister, it appeared at the beginning that she was working in the home and then later scenes she was never at home supporting her extended family, so where do people think she is during the day and through the night?
There were a LOT of unanswered questions. Very frustrating and incohesive.

there is no need to tell the story of every family member. that happens quite often in films.

maybe the cyclo by himself didnt know the relationship between the woman who owned the "bike" and the syndicate. so why should we? its just he owes her money and they were able to make everything the want to do. or at least, as long as he has not payed the debt.

i think she was really the boss of the syndicate, though thats quite unusual for mafia-like organisations.

and i think the guy who has stolen the bike was one of her guys.
she knew that if they have no "bike" anymore the boy has to do what she want.
another assumption: without the boys money the sister wasnt able to get enough money for the family,
so she had also to sell her body.

think, that could be a somewhat usual method to get into control of other people

the boy of the woman with the rented bike was also in bed with the poet,
so that seems to be a hint.
and it that scene she also tells the story why her boy is mental ill,
when he was born he came out with the feet first,
so he had not enough air and thats why he is ill.

that are my interpretations ^_^