As you said, northeast
1: Electric Field Diagrams. It's a standard rule to always make arrows point from the negative charge to the positive charge. This is to probably indicate the electron flow.
2: The average charges of the positives are in the p direction. You know that because if you look at the diagram, the +qs are pulling the -q to the right and up. So it's averaging out to be pulled up at a diagonal. (Use the Pythagorean theorem or trig identities if asked to do the math. It's essentially basic forces, but with particles instead.) So the + force is at p.
So if the force is between p and -q, with p as the + force. And, as stated in 1, just draw an arrow going from negative to positive.