Is it possible to turn mid air, and land a jumbo Boeing 747, Airbus A380 or any aircraft if the vertical stabilator is jammed in a position that allows it to fly straight at cruise altitude and speed, assuming everything else works including the horizontal stabilators?
If the rudder is jammed in the neutral position (as stated in your question), then landing is easy. You will just land using ordinary techniques. The plane may feel sloppy, and passengers sitting at the back will complain about feeling dizzy, but the aircraft is easily controllable and landing would not be any problem.
Now things are different when the rudder is jammed to one side, for example in American Airlines Flight 96. The crew faced major controlling issues with a full right rudder, engine failure and very high sink rate. But even then, the pilots managed to land successfully.
Yes. One does have to be careful in turns and do it gently but it can be done as this graphic example of a B-52H on a test flight demonstrates.
Honestly a rudder failure during cruise flight does not present a major risk but would have to be handled with additional precautions such as gradual, gentle turns at higher speeds and a reduced flap setting landing in the airplane. Additional differential thrust using the engine thrust levers can compensate for some of the adverse yaw encountered at these times. The yaw damper would have to be disengaged as well.