Thats a pretty light duty machine. Keep in mind that unsupported rails will always have a bit of sag and flex.
3) I like to call that the X-axis, with the Y-axis being the axis on the gantry. I think you're right, without beefing up that gantry quite a bit, it would bind if not using 2 screws. Those little black boxes appear to be the stepper drives.
That's a nice machine for doing engraving or balsa work, but not really designed for anything heavier.