There are different components to the motion of the ball and maybe this is where you are getting confused. Consider the demo with the ball being passed backward over the head yet still travelling forward as thesis the simplest part of the concept.
Similarly, Red 13 passes backward legally to Red 14, but as they are both at pace the ball actually travels "forward" - This is not "thrown forward"
When Red 14 fumbles the poorly passed ball and it
traveled backward ie fell behind them I would consider that play on - nothing to see.
If it is fumbled and falls in front, as they are still running up the pitch then it has travelled
towards to the Oppo's DB - "knock on"
Again the definitions are there with my emphasis:
Knock-on: When a player loses possession of the ball and
it goes forward, or when a player hits the ball forward with the hand or arm,
or when the ball hits the hand or arm and goes forward, and the ball touches the ground or another player before the original player can catch it.
Forward: Towards the opposition’s dead-ball line.
Really not sure how to try and explain this anymore

I feel like I am trying to explain a simple attacking 2v1 in a coaching session and the basic principle, of fixing the defender, is not being grasped.
What do you do in a game?
Do you get berated for the decisions you are making?
Do you hold off making decisions because you are unable to decipher what you have seen?
If no and no then I would stop worrying and move on.