"She was not an actor."
Translation:Níorbh aisteoir í.
If the object didn't begin with a vowel, would this just be ni raibh? Ni raibh cocaire i?
No. This is the conditional/past copula, not a form of bí. It'd be Níor instead.
Not always. A more complete answer is, "it depends". It depends on the word and the region whether it will be a V sound or a W sound. If the word ends with bh, there's a good chance that it will end up having a V sound, like Niorbh.
It could be Ní raibh sí ina haisteoir (though I don't know if that's included in Duolingo's list of acceptable answers).
ina haisteoir because words that start with a vowel take a h-prefix after the possessive adjective a when it means "her". "He wasn't an actor" would be Ní raibh sé ina aisteoir.