1) LOC appr. is always selected(vertical profile)
2)Never arm appr p/b
3)Easiest way is to arm loc and let the a/p look after lat. nav. so you can fine tune FPA
4)you may not arm loc p/b,use trk/fpa to select vert. and lat. nav but I cant see the point in this.
TO answer your Q. use either but 3 is just the easiest way