If you roll high enough to knock the target back, say, 15', do you have to move with them the full distance, or can you knock them back that far but only move a portion of the distance (or none at all)?
If your attack is successful, your target is pushed back 5 feet. For every 5 by which your attack exceeds your opponent's CMD you CAN push the target back an additional 5 feet.
The text says 'can' so you can elect not to push the full distance!
If your attack is successful, your target is pushed back 5 feet. For every 5 by which your attack exceeds your opponent's CMD you can push the target back an additional 5 feet. You can move with the target if you wish but you must have the available movement to do so. If your attack fails, your movement ends in front of the target.
So you don't have to move with the target at all if you don't want to.
Yes. The word "can" makes it optional (beyond the original 5 ft push, which isn't written as optional).