The huge challenge is that the whole expression has to be carried by 3x3 pixels per eye.
The three with (nn) (oo) (xx) work pretty well, others are not as immediately readable.
For the crying one, I'd put tear eruptions on the sides of the helmet. Same idea for an angry smiley: red zigzaging lines in the 'background' would convey the emotion. I don't know if animation can be used.
A more radical change, maybe reducing the contrast between face and helmet would help : it would avoid interaction of the '3d edges' lines with the pixels of the expression.