He can't move if he's caught in genjutsu but his sand is still working and let me doubt if that's as easy as you're making it sound. As said before he could use the third eye which would give him a wide view over the field.Consequently closing one eye in the proper circumstances doesn't leave you wide open.I'd like also to remark that Gaara needn't close his eye in order to use it.
Shukaku had nothing to do with the sand.
