Translation:Which country is Maria from?
I think that still needs another 'from': "From which country is Maria from?"
I interpreted it as a question, "Maria, what country do you come from?" Which should also be correct depending on the context.
Not necessarily. Honorifics are often dropped between friends, family, and if a foreigner has made it clear they prefer being addressed without it (me～!) However, it is a little incongruous with the sentence using です, though scenarios exist where the honorific is not strictly necessary.
Strictly speaking, it should be: "Which country was Maria born in?"
Almost everyone I've met asks 「どの国から来ましたか？」. And when I reply with one of イギリス, 英国, or 北アイランド, they would then typically go:「ご出身は？」。
In this context, the correct translation is: "Yes, but where are you really from?"
Exactly. Usually I hear, 「どの国から来ましたか？」Then if hometown (出身) is mentioned, they want to know which city in your country. This one confused me a bit because I thought it was asking about Maria's hometown. I got native confirmation that some people use this phrase to ask which country someone is from.
That's interesting, I've had the opposite experience where everyone always asks me どこの出身 (doko no shusshin), but I had only ever learned から来ました (kara kimashita) at school and it took me forever to learn and understand the word 出身 (shusshin).
出身is not only used for the country of your origin. It can be used for the school you attended. E.g. 東京大学出身です。
Why is there the particle 「の」between 「国」and 「出身」? I know in a response you wouldn't use a 「の」(For example: マリアさんはロシア出身です。）as tought in the first lesson, so why use it here?