Depending on the context/situation there could be a difference in meaning. A victim has suffered some injury as a result of something someone else consciously did: murder victim, robbery etc. A casualty suffered an injury -- physical, psychological, financial, -- as a result of some natural occurrence -- flood, tornado, even war. Another distinction to me at least is 'casualty' implies the person survived, 'victim' implies the person didn't. Let me add though that these are just my impressions and other users may have a completely different take on it.
That should be OK. It was probably wrong because nobody has thought of putting it in the list of correct answers. http://www.wordreference.com/iten/reverse/vittima http://context.reverso.net/traduction/italien-anglais/vittima
It depends on the lesson. I don't think Duo accepts "yarn" for "la lana" yet, even though it is the preferred word in the US for "string made from wool". Also, "lounge" in the US is usually an area for relaxation in a hotel, airport, workplace, or even some kinds of bars, but Duo always shows it as a living room in a house, which I hear is sometimes called a "lounge" in British English but not in American English.