My cables on my AL-Ko chassis, go into the top. If this is correct way around, Just make sure it's mirrored on other side of the axle, and orientation.
Only on the caliper fixed and bits the piston end to the shoes and any shoe rubbing points, "Copper slip " high Temp grease should be used , but i think if you have taken the brakes apart I think you know that. The brake shoes are non reversible, unless Murphys law applies.
Getting back to your question the leading edge is the one raising into the the direction of rotation, dragging the shoe into contact.