to hit the ball back i reckon that you just us M1 to use the attack as usual and it will hit the ball back. it might be M2 to throw the ball like with the pyros air blast then just hit the ball
I have a feeling that the normal swing will be M1, ball hit with be a charge up and then hit with M2. The taunt will be the kill taunt. You likely won't be able to hit the balls back with your bat, but rather use your Force-A-Nature to repel them back at the original scout.
thats the exact same thing as i said.
i believe the stun gets longer the further the ball is hit
Not quite. As I said, for the bat, M1 would be a normal attack, M2 would be the ball attack. You wouldn't have to manually use the bat to hit the ball, it would be automatic (like any of the guns). For repelling the ball, you wouldn't hit the ball back with a bat, you would have to use the Force-A-Nature (third unlock).

One thing i notice though is that Beanball the achivement for that has a BAT hitting the ball back at the scout in the picture not the force-a-nature, it makes more sence useing the bat anyway when you think about it
EDIT: and also going in and looking at it its the standard bat doing it, i wonder if the sandman cant hit balls back?
If you're able to repel a baseball, whether with a bat or the Force-A-Nature, what else will you be able to bounce back? Stickies? Grenades? Rockets? I don't know, but unless it's limited to just baseballs, it seems overpowered.
i smell a baseball custom map..