What makes you think this fireball jutsu will be activated BEFORE Kamui? Naruto was near full speed when he was warped. It's not quick enough, and it won't cover the distance.
This is going by the fireball being quicker than Kamui, which is untrue.
Again, you need to realise Naruto's speed.
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Maybe you cannot see it properly in the manga and the anime will make it clearer for you. Itachi is not on that level of speed and he will not be able to close the gap before Kamui is used. Kakashi can also make clones aswell. Kakashi clones are not unbeatable. These things are not happening before Kamui is activated.
What can KA do? He can't use it to make Kakashi kill himself. If the command is not subtle it won't work, because Kakashi will no he's in the genjutsu and that is enough to break it. You aren't taking these things into account. Again, Kamui's activation is faster than all of Itachi's methods, and since he has no way to dodge or counter it once it's used, he loses.