1- to do this you need to take a good look at your dish, the feedhorn, and the travel it makes across the ARC. Now use your imagination and look at the ARC and the way it would travel through the center of the feedhorn. You would place any extra lnb's on this imaginary line, on one side or the other of the feedhorn. Whichever works best for you.
2- assuming you have a BUD, the polar mount should be pointing true south/north, not magnetic, but true. Not sure what you mean with "sync my motor", but when the polar mount is pointing true south/north, you need to have elevation set to your location/latitude. Then set the declination for your location, and the dish should travel the ARC correctly.
3- it doesn't have to be exact, but the more level/plumb it is, the better off you will be.