If you think about it he would need to be super fast but if he could move the sand into the path of Kamui, the sand would get absorbed and not Gaara.
Same thing with Amaterasu. He can use the sand as a shield to stop it touching him. It's not the same as a normal object. Gaara's sand is full of chakra. These are my opinions and I base them as closely as I can to the anime & manga.
it blocked already amaterasuu
it works automatly , because there is Gaaras mothers spirit in The sand So IT defends gaara without doing something ..
Shiled of sand is Like Skin , which covers Gaarsa body ..