In this example it doesn’t make much sense to talk about north, south etc. I would say ‘…at the top of the diagram/plan’

Either could be. There's no way to choose between them without more context.

Either might be correct. Do you have the entire sentence?

At the North End , Because At is used when we have to show a place location. In is used when qe have to show what's inside something.

At the north end