Something I thought was weird was that Voldemort was using Nagini as a Horcrux. I think it's bizarre to use a living thing as a container for one's soul. What happens when the snake dies? Is the bit of soul destroyed? Is there a chance to put it somewhere else? Does the snake live forever, so long as the other Horcruxes and Voldemort are intact?
I thought that was odd too. Dumbledore just suspected that it was the snake, but yet, he admits that he could be wrong because he is human. It may not be after all. I would think the soul would die with the snake, if it died. Her death would mean the horcrux is destroyed, right? I don't think Nagini's life is at all tied to the other horcruxes. They seem to be totally independent of each other. Voldemort does not even know when one is destroyed, according to Dumbledore's beliefs. I don't think Nagini's life is tied to Voldemort's life either since Voldemort is also a horcrux. Nagini being a horcrux means she is able to somehow be used by someone to bring Voldemort back to life if Voldemort (that is his 1/7 of his soul) is killed. I would think that if she was one, it would have been easier to use her in GOF to get Voldemort back his human form rather than Harry. Though, Voldemort was just trying to kill Harry, and didn't want to waste a horcrux, perhaps.
I gotta go workout. Maybe that will clear my head............oy!
Yet when Harry was having the dreams in OOTP, he "was" the snake. He saw things from the snake's point of view. That would indicate that VM was "riding" Nagani at the time. Prior to the arrival of Wormtail, he was inside her all the time.
Then, he killed that other witch (forgot her name) thus completing the necessary murder for the seventh split, leaving part of his soul in the snake. It stands to reason, therefore, that he could more easily take possession of Nagani's mind if part of his soul is already in residence.