Is cruise ship and merchant ship able to backward without tug boat to assist?

Yes.  Most cruise ships have azipods so they can steer by computer going backwards.   Cargo ships go backwards just to keep from hitting stuff.

What is interesting is you would ask that question.

Yes.  Most ships can do backing easily with a reverse of the main engine.  There are also side thrusters in the bow to help steer.  If there are two or more propellers in the stern, left and right side ones can go in different directions to help change which way the stern will go.

Modern cruise and merchant ships have "Azipod" systems that are fully steerable propellers that can point in any direction.  This lets the helmsman change the thrust direction to any heading in a matter of a few seconds.  The propellers are mounted on geared pods in the stern. 

Most cruise ships dock without assistance in small ports.  The large cargo carriers can do much the same.

In an emergency, yes, But as a typical manoeuver, no.

Yes, they can move forward or stern-ward. The engines can turn the propellers in both directions