Concerning the scrolling: you can actually get away with only 2 background objects like this:
The world just moves the background left and right, and the background always checks weather it is within bounds. If it is not it will adjust itself and switch sides. And because the code uses "getObjects()" we don't need to save the background objects into variables / an array.
// In world constructor
addObject(new Background(), 0, getHeight() / 2);
addObject(new Background(), getWidth(), getHeight() / 2);
// In world
@Override
public void act() {
if(Greenfoot.isKeyDown("right")) scroll(SCROLL_SPEED);
else if(Greenfoot.isKeyDown("left")) scroll(-SCROLL_SPEED);
}
private void scroll(int speed) {
for(Background b : getObjects(Background.class)) b.move(speed);
}
// In Background class
@Override
public void setLocation(int x, int y) {
while(x < -getWorld().getWidth() / 2) x += getWorld().getWidth() * 2;
while(x >= (int)(getWorld().getWidth() * 1.5)) x -= getWorld().getWidth() * 2;
super.setLocation(x, y);
}