If a player is running north with the flight of the ball and gets a hand to it and knocks it south but not hard enough to change the direction of the ball then surely that is still a knock on despite the ball not actually changing direction.
Then you need to focus on the direction in which the players hands face, their own goal line or the opposition's?
Then you have people 'facing backwards' who can still knock it forward purely down to the balls momentum and 'bounce of the ball'.
Think of cricket, a batter hitting the ball straight down the pitch, bowler reaches out and gets a finger to it and it goes into the stumps.
The ball was only ever going in one direction , it's speed and trajectory was merely altered (knock back).
Incredibly unlikely it goes forward then backwards again into the stumps (knock on).
For me, it's as simple as did the direction of the ball go towards the opposition try line at the point of the touch.
If you are lucky to get a finger to a ball that's speeding past you and it's a glance and no direction change, play on.
Refs being consistent is another thing.