No, you don't need vents on all walls. You do want it on more than one wall, for cross air flow. But if one side faces more wind than the others, it's fine to have that as a mostly or completely solid wall.
I would put it up higher in most cases. You get more benefit with venting up higher, as ammonia and warm, moist air rise, than lower. In larger coops you'd start to consider things like circulation of air throughout the space so letting in air low would help lift and push out ammonia and such through higher vents, that sort of thing.