1. Do the formulas for the mid-point and length of a line joining two points need to be memorised?

No, although they are quite easy to learn, they are usually provided in tests and examinations.

2. When finding the length of a line joining two points does a formula have to be used?

No, if you draw a diagram, it is quite easy to use Pythagoras theorem without using a formula.

3. Does it matter which point is POINT 1 and which point is POINT 2 when using a formula such as the one for the mid-point of a line between two points?

It does not matter as long as x1 and y1 both come from the SAME point and x2 and y2 both come from the other point.