Is "van" necessary?
Yes, "van" is necessary here as the question is about location and not about attributes (predicatives).
Where is the girl: A lány hátul van.
How is the girl (like): A lány szép
What's wrong with "A lány elöl van, a fiú meg hátul van?" I was marked wrong.
This is grammatically perfect, but Duo seems to prefer parsimony, and in this case you can omit the 2nd 'van'. Actually the sentence is more natural without the 2nd 'van'.
This was marked wrong, 'meg' should have been 'pedig'. Somewhere else here it says 'meg' and 'pedig' are interchangeable.
Why does meg goes after a fiu? It sounds to me as "the girl is in the front, the boy and is in the back
Can I use pedig instead of meg here?
Yes, you can!
Is it wrong to use és here? The English doesn't really distinguish.
No, not at all, "és" is fine here.
It's not wrong per se but you should use "meg" if you really want to express the same thing.
Is there a reason why meg is preferable, apart from the fact that the two of them are in opposite directions?
"meg" has nothing to do with directions, it's just the way to express this in Hungarian.
Alright, thank you!
What is the difference between meg and pedig (and es while we're at it)?
In this context there is no difference, they are interchangeable.