A pusher axle is the rear drive axle. A Tag axle is the (normally) unpowered axle. A drop axle is one that is only deployed when the weight conditions warrant an extra axle. Tractors that routinely haul nose-heavy loads use them but they are not standard equipment on tractors.
Those spiky things are legal all over the US, as they are only covers for the lug nuts, They are made of plastic or very thin aluminum. I have accidentally dented them with my thumb. I cannot speak for other countries tho.
