No.
There is no such thing as an EU citizen. Each country of the union has it's own laws and governance over citizenship.
You have to become a citizen first
Nationality can be acquired through birth in a country, descent from a citizen, marriage to a citizen, or through a legal process like naturalization. Each country has its own laws and criteria for determining who is eligible to become a national.
Rihanna is an american african citizen since she was a young girl.
Yes. Marriage does not depend on citizenship to be maintained.
it is illegal
It is not possible to become a Canadian Citizen just by marrying a Canadian citizen. Citizenship cannot be acquired by marriage in any country.
No, it would be considered a marriage abroad.
No, You can only become a citizen if you apply for you citizenship through immigration. Being a legal resident and married to an American citizen can and will not change your immigrant statics unless you go through the right process.
No they do not have to be born a US Citizen. They can become a citizen through the naturalization process.
The answer is no. But the children born from this marriage will automatically become US citizens. The spouse of US citizen will be eligible to become US citizen after he/she (the non-US citizen) stay in US for a minimum of 3 years as a Permanent Resident.