'Czekam przed wejściem do kina' should be also correct
Wouldn't that be more like "I am waiting in front of the entrance[…]"?
Strictly speaking yes, but I would accept it.
"Obok wejścia"? Can you just say "przy wejściu kina"?
I just added "obok wejścia". I don't think "przy wejściu kina" is ok, though. Google shows some results, but very few of them actually mean "entrance to the cinema".
Yes, "obok wejścia" makes sense, added. Although technically it's "next to".