"Which country is Maria from?"
I know that it is fine to say (country) しゅっしんです to indicate where you are from, but why do I need to add the particle の in the question when の is not part of the declarative sentence?
I think that the particle の implies that we must answer not only the country but also the city.
Q:マリアはどの国の出身ですか。Where is Maria from?
It is not so important to be so precise, but literallyどの国 means "which country" only. Thus merely answering the country name is not wrong.
Typically surname or full name is used with 'san'. This sentence is used personal name. I do not know the situation. I think it is good that you do the same as everybody there. Or depends on the relationship with 'Maria'. This relationship is not status, but friendship and love. Or depends on character of 'Maria'. it is good you use 'san', if you worry about. 'San' is not special. Adults use to each other.
this is my personal opinion.
It depends on the situation. This is fine for example, the General Manager asking a Manager about his new intern Maria, while Maria is not there.
For the generic term Country, Kunyomi Kuni is used.
I wrote「マリアの国はどのですか」and was marked wrong. Why is this incorrect?
As you use の it should connect to a noun and cannot stand alone by itself. Otherwise you should use the Interrogative pronoun どれ.
Just a side note マリアの国 may be not a natural way to say (as it seems like she owns the country). I guess we need to say マリアの出身国.