He wouldn't know Mina to or edos are there.
I'd like to see him do that but he kamui's back above naruto, and as soon as he's about to hit, minato ftg's him. "Sensei??"
I don't want kakashi to die either but how will he beat obito if he put a hole through his chest and it didn't faze him. Plus if someone's kamuis another's body part he could just pick it up and reattach it where its in the same dimension.