"She looks cute!"
Yes, that is another way to express the given statement. 看起来 has extra emphasis in feeling and is common to use in practice.
Not unacceptable, but it's more of "She's cute (in the adorable sense of the word)".
Is 长得 really necessary here? I was marked wrong without it, but 他很可爱 seems to express the same thing to me...
I can see it as a set phrase in pleco though, but the only example is very similar to this one.
Chengde is not necessary, and Ta hen keai is acceptable. If you report your translation as "should have been correct" they eventually change it. (Sorry for not having a Chinese keyboard
"is cute" and "looks cute" are very different grammar structures both in English and Chinese. For once Duolingo is right not accepting the former. Are you guys trying to learn a language or win a quiz? :)