this is an awesome problem and there is a good concept behind it............

first of all the normal reactn DOES NOT pass through the centre of mass in this case. ill tell you why........

the block is coming down with uniform velocity which meas that the frictional force = mgsin$.........

but if we closely analyse from the rotational frame it is clear that the body is not rotating about its centre of mass but is simply sliding down

this means that the net torque on the body about its centre of mass=0

here if observed closely the there is a torque provided by the frictional force acting at a perpendicular distance s/2 from the centre of mass of the block(since it is a cube) Hence there should be a counter torque to cancell out this frictional torque which is indeed provided by the rough plane by shiftingthe normal force by a distance s/2 from the line of the centre of mass thus to provide a torque mgsin$ *s/2 anticlockwise to counter the frictional torque....

thus it is not always necessary for the normal force to act along the line passing through the centre of mass of the body as seen in this case....

rate me if you found my expln satistactory or correct me if i were wrong.....