your questions are easy bro. adam lived for 930yrs, in that period he must have had thousands of children and grand/great grand children(gen 5vs3-5) so some of adam's other children or grand children could have heard the story of the murder of abel by cain and might want to revenge.

then about his wife, the bible account didn't say he met his wife in the land that he ran to, rather the account states that he had sexual relation with his wife, implying that they were already married(brother&sister) before he was banished.