Your answers to questions 1 and 2 are correct, including 2b - very good!

As for question 3 - I suggest you start with sum of forces =ma for the boat + barge system. The sum of forces is the driving force of the engine minus the resistance forces on both boat and barge. From this you can calcualte the acceleration of the boat and barge system. Now apply sum of forces = ma for the barge: the forces acting on it are the tension in the rope minus the boat's resistance force. Its mass and acceeration you already know, so you can solve for tension.