Gaara can't make his sand lighter or heavier... Oonoki is the one who did that. and while the sand on its own may not attack, it's defense is just fine. for an example look back to when Gaara was attacked by Yashamaru when he was just a child. Gaara had little control over anything at that time but the sand completely neutralized the attacks of Yashamaru, a Sand Anbu shinobi. as for the raikage/sasuke thing, it is true that gaara controlled it but that was both an attack and the defense of another person, not of himself. gaara controls sand all the time, especially to attack, but the sand does protect gaara of its own accord