Sadly, no.
US citizenship.
If he is a US citizen that happens to be in Turkey, then yes. If he has Turkish citizenship, he will need to attain US citizenship (either dual citizenship or give up his Turkish citizenship for an American one).
Everything you need to know is at the U.S. Citizenship and Immigration Services website: http://www.uscis.gov
Yes, but you would need a visa or have citizenship from a VWP participant country.
If you are running for the US Senate, you must be a citizen of the US.
You don't need citizenship in the US to drive, you do need a valid driver's license from a country that is certified.
No. The green card should have minimum six months validity before you can apply for US citizenship. If green card is about to expire you need to apply for its renewal first and then apply for US citizenship.
How you acquire citizenship is irrelevant. You simply need to apply and pass the fitness, health and background tests.
Your friend don't need a citizenship if he is just on a vacation.
If you apply for citizenship in another country your US citizenship is automatically revoked. The US does not recognize dual citizenship. You revoke it in writing at a US embassy, outside the US.
Yes, The residents of US territories have US citizenship
The US recognizes dual citizenship.