Q1 Why is work done by friction path dependent?
Q2 Why work done by non-conservative forces is equal to force * distance instead of force x displacement?*
Since friction forces are non-conservative, this will answer both questions.
Friction work is non-conservative because it is dissipative (generates heat). The friction work is the friction force (assume kinetic friction) times the distance covered in the path from A to B. The greater the distance covered in the path between A and B, the greater the friction work and the more the heat generated. What’s more, if the mass is returned from B to A, the total work done will be the sum of A to B and B to A, and will depend on the total distance traveled. If the mass begins at rest and ends at rest, all of the work done generates heat.
Contrast this to the work done in a gravitational field. Gravitational force is conservative. Let B be at a greater height than A near the surface of the earth. The only work that increases the potential energy is the force times the positive vertical displacement between A and B. Any horizontal displacement involved in the path between A and B does not change the potential energy. If we now return the mass from B to A, only the total negative displacement will result in a decrease in potential energy and it will be equal in magnitude to the increase going from A to B. If the mass begins and ends at rest, the net work done is Zero.
Hope this helps.