1. Change the cameras x/y position as you did with the view box:
function move_camera()
{
camera.tilt = 90; //turn the camera to the bottom
while(1)
{
camera.y += 1*time_step; //move the camera
wait(1);
}
}

2./3. Use vec_for_screen to get your screens frame:
action player
{
var screen_pos1[3]; //vector definition
var screen_pos2[3]; //vector definition
screen_pos1.z = camera.z-player.z;
screen_pos2.z = camera.z-player.z;
while(1)
{
//movement, ...

screen_pos1.x = 0; //upper left corner of the screen
screen_pos1.y = 0;
vec_for_screen(screen_pos1,camera); //get the world coordinates

screen_pos2.x = screen_size.x; //the screen corner on the right bottom
screen_pos2.y = screen_size.y;
vec_for_screen(screen_pos2,camera); //get the world coordinates

clamp(my.x,screen_pos1.x,screen_pos2.x); //clamp
clamp(my.y,screen_pos1.y,screen_pos2.y); //clamp

wait(1);
}
}