i) You need to be a US citizen to apply for your fiancée as a K-1. If you are a legal permanent resident, you cannot file a petition to bring your fiancé/fiancée to the United States.
ii) You must intend to marry your alien fiancé/fiancée within 90 days after he/she enters the United States. After 90 days have lapsed, the K-1 visa will no longer be valid and your fiancé/fiancée will be considered “out of status”.
iii) You must be legally free to marry.
iv) You must have met your fiancé/fiancée at least once within the past two years (unless you meet one of the exceptions outlined in the special K1/K2 visa scenario section)
v) You must prove that you can support your beneficiary financially.
vi) You must submit a copy of your criminal record if you had been convicted for a crime or crimes.

Yes because he is now a US Citizen and he is travelling with US Passport. Upon your return back to the US, US Immigration officers inspect your US passport to check if it is valid. They don’t ask many questions to US citizens unless they suspect criminal or suspect you obtained your status through fraud or think you traveled to a country they consider a terrorist country or your US passport is invalid. If your US Passport is ok, they let you pass the custom and immigration since you are a US citizen and US is your home. Immigration officers do not deport US citizens since US is their home.
 However, this could also trigger immigration officers looking deeper to check whether you lied on your application (obtained your status through fraud). think carefully and take your own risk if you decide to travel to a country you seeked to be protected from. talk to your attorney before travelling.