1. If you have a boat in the area, they can't trade through it or use it as a link in a naval chain for invasions. That's all there is to it.

2. With successes. Can always make em pick of bad generals at home before sending them out, since it's usually border towers that cause assassins to get caught. Do you have VI? If not, you'll probably not get too much use out of assasins, simply because getting one to a usable valour it so hard.

3. I have no idea.