Lets get your shit straight then.
If headband can casually slip off then extra force applied to that headband can knock it of. Its physics axiom so to speak.
Your mistake is thinking it can casually slip off. It didn't. It slid off due to Itachi's head being bent down. You can't hit a headband with a kunai and knock it off. Seriously try to picture it happening frame by frame. not possible.
Let use say hat as example, if it can fall off then if i hit your head it will fall of a lot easier.
That makes no sense. Are you really thinking about this? If you hit the hat you will only be hitting my head and the hat.
The kunai didnt miss either, if it did it wouldnt be in same scan as it would flew by way behind.
Yes it did. It even landed before the headband as shown by the impact marks
Im sorry you dont get it, but there is nothing i can do, i myself could provide better counter-argument then truth.
The fact is he is correct and you are not. Your bias is not allowing you to see it. It's one thing to dislike a character(I dislike Itachi myself) but It's another to allow your feelings on the character cloud your judgement.