Re: Which cable stops reverse from working?
Looks like the bottom one is the shift cable. Have someone move the remote handle forward while you watch the engine. One of the cables (I believe the upper one) should cause linkages to move and the carb throttle plate to rotate (do this with the engine off). Once in gear (either forward or reverse) the prop will "lock" and you will not be able to spin by hand (again, do this with the engine off).
Once you figure out which one is not the throttle, with the engine off and control in neutral, loosen the clamp securing the cable at the engine (can't see how it is attached from the picture, but disconnecting the shift cable from the linkage would be best). Actuate the metal bar forward and reverse - the prop should lock in both directions and free spin in neutral. If all looks well at the engine, you likely have a bad cable or a problem at the remote.