What is the interpretation of a dream about jumping on someone's back and grabbing their neck, and do I still carry a sin towards my cousin's son after refusing to marry him, performing Salatul-Istikhara, and the emergence of problems with a fellow student?

Interpretation of dreams is declined as it is a profound science to which only its specialists should attend. Your dream is not necessarily connected to your story with your cousin. Your refusal to marry him is not considered a sin, as accepting a suitable partner is recommended, not obligatory. If your cousin is a man of religion and good character, and you are not yet married, and you hope for a lasting companionship, then accept him as a husband and repeat the Istikhara prayer. If it is good for you, Allah will facilitate it.