1. What is the difference between an axis of symmetry and a mirror line?

Not much! A mirror line is part of a reflection. An axis of symmetry is usually drawn on a shape.

2. Can a shape have an order of rotational symmetry of 0?

No. All shapes have an order of symmetry of at least 1 (This is a turn of 360o)

3. What is point symmetry?

Point symmetry is when a shape maps onto itself under a rotation of 180o (a half turn)