twice the same sentence!
Kell oda az a vessző! Ez felsorolás.
Can you help me understand when to use mögött and when to use hátul?
mögött - behind something. Like az ablak mögött=behind the window.
Hátul is generally in the back. (not compared to some other object) The boy is sitting at the back. A fiú hátul ül.
Would vékony work here as a synonym for keskény?
Not here. A line of paint, a hip of a woman, a small creek can be both but "vékony" is more like "thin". Usually for objects where you want to fit or go through (street, door, window) "keskeny" is used.
So in English it's like thin vs. narrow?
Sorry, could someone tell me if it's mandatory to use "vannak" on this sentence or not?
Yes. When talking about location you need to use vagyok, vagy, van, vannak, etc.