well, if i recall correctly, his sand moved independantly of his own thought most of the time because didnt his mothers spirit or chakra or whatever possess the sand? If thats the case couldnt shadow possession take control of the inanimate sands consciousness as well? I say this, remembering how back then during gaaras fight with naruto, the sand kept up with naruto even when gaara couldnt which first showed that it was him 100% controlling it, then i thought i read another issue later showing how gaaras mother put her chakra into the sand to protect him.