The diary started controlling/possessing Ginny when she wrote in it. If he would have completed the transformation, she would have died. I would think that DD's hand turned black when he put on the ring (horcrux) because the ring started to control him and Snape had to kill the hand and destroy the ring to get it to stop. I would not want to be the one to drink from Hufflepuff's cup or put on the locket.
I think he is dependent on someone opening the horcrux but then he takes over and kills them in the end. I am not really sure if I am right, but this is my understanding of how it works.
Makes sense, yes, he needed Snape's help to destroy the hand/ring.