1) The trebuchets can be fired from inside a castle. They can be useful in disabling enemy siege weapons. They are rather hard to position, however. They are not very useful in a field battle nor should they be. I don't recall ever being able to penetrate any walls without ladders, making a breach or breaking a gate.

2)IIRC, command stars improve attack, defense and morale values. They do stack with other traits.

3) Each sword increases attack value by one.