"The boss stands up from behind that table, under which there is no carpet."
Translation:A főnök a mögül az asztal mögül áll fel, ami alatt nincs szőnyeg.
Yes, in fact, it works even better (by a little). It should be accepted.
"amelyik" works best when there are multiple tables, and you are talking about that table.
"amelyik" does not work so well when you want to say a table, under which there is no carpet.
These aren't strict rules, though.
Sometimes “behind” seems to be translated as “mögül”, but sometimes as “möggűl” or so...What does that mean? What is the difference between those words?
I suppose you mean mögött and mögül. Mögött simply means behind, while mögül is from behind.