Adam as and Eve lived for 1000 years. Enough time to build villages or even small cities. We do not know how many people were around at the time Cain committed the murder.
And I know nothing about Cain fleeing to a city in Islaam.
About his wife
Adam as and Eve always had twins one boy and one girl. back then, it was allowed to marry the twin of your brother but not your own twin sister.
So, Cain had to marry Abels sister and vice versa.
For what I know was Abels twin sister Cains wife. I do not know whether Cain had a second wife elsewhere, or later on.
