Why does "Na nás se divá?" not work?
It is a strange word order for a question to start with an object. At least in this case.
Just strange or also wrong?
It seems like it would work if you already knew he was looking at someone/thing but you were emphasizing the question of if it was your group that he was looking at.
Wrong, if your aim is to learn how natives read and write. If you want to emphasize nás, place it final and stress it when pronouncing.
If you just want to argue with some author of some exam, we can't help you with that.
"Pozoruje nás?" - "Is he watching us?"?
Nevím, jestli je pozorovat totéž co look. Každopádně by bylo dost práce to dát do všech vět o koukání. "Is he watching us". je akceptováno.
Of course "On se dívá na nás?" would be also correct, wouldn't it?
Yes, "On se dívá na nás?" is also accepted.
Ve větách předtím jsem měl že looking je hledat.
Hledat znamená "look FOR / search FOR."