One way to limit the lag by far is to keep the part count low. Try and add larger parts as opposed to a bunch of small ones. Also I've noticed it helps the lag if you point the camera away from kerbin. Looking at the planet makes the game have to render it. If you are facing away from kerbin or wherever you are the game will play a lot more smooth.
Think about how hard it was for NASA to land on mars. I do not think that they would be able to bring anything big to kerbal if they got there, let alone bring anything back. There is no way they would be able to attack kerbin in any way shape or form.
I believe that they would share culture, technology, and mathematics. Humans nor kerbals have the resources to fight each other from a crossed solar systems.