Wrong. Kakashi clearly saids he couldnt properly control the location and size of kamui.[I guess it depends on your definition of "defended". Deidara was able to move his body out of the "wormhole" and restrict it to only his arm being hit, where as the databook stated that nothing can resist being pulled in. It was defended in the sense that it didn't hit what Kakashi was aiming at, and Deidara was able to move out of the way. It was originally aimed at Deidara's chest/heart. It ended up taking his arm. If Deidara were faster, it wouldn't have hurt him period.
That was an "imperfect" version of kamui as Kakashi has improved it..but that databook was released shortly after the fight with Deidara, and prior to any other kamui feat.
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]Nope disagree. Kakuzu's blast consist of more than 2 elements. If it was fired at YM in a single shot the mirror would repel it.Every Jutsu has a weakness. Yata Mirror isn't a jutsu, its a spiritual weapon. And it has a weakness, Yata Mirror just changes its attributes to repel an incoming attack, two attacks fired at the same time with different elements should bypass the mirror, and the Mirror also doesn't protect from underground attacks.