Question: Can vampires and werewolves be killed or injured by anything other than vampires and werewolves?
Answer: Sure. The problem is that these films portray vampires and werewolves as having super-human abilities, so it'd be significantly harder for a regular person to kill one. But nothing about the films seems to indicate it's overtly impossible.
Question: Can someone explain what it means that Jacob imprinted on Renesmee? I thought imprinting meant you want to be with that person (like love and marry them). But now that Jacob has imprinted on Renesmee, I think I'm wrong.
Answer: Werewolf imprinting means you love that person in exactly the way they need you to love them. In Jacob's case, this means he feels a fatherly/brotherly love for Renesmee now. When she gets older those feelings may change to a romantic love, but that depends on whether Renesmee is ever able to see Jacob as anything other than a big brother. If she thinks it's too weird to feel romantically toward Jacob due to the nature of their relationship during her childhood, he will forever be her platonic protector and be absolutely elated by it. Fast as she grows though, it's unlikely they'll have spent so long in those familial roles that she won't be able to see him as anything else.
Question: What if the Volturi died? Vampires would end up killing all humans, right?
Answer: It would be rather difficult for the small number of vampires that exist to kill six billion humans. Even if they could, they would be eliminating their food supply. As to the Volturi, it's unlikely all three would die and at the same time. It's probable that a line of succession is in place to replace them. There are other vampires working within the Volturi organization that we don't happen to see.
Question: Why does the baby's face look so weird?
Chosen answer: It's because it's a computer generated image. Renesmee, Edward and Bella's daughter, grows almost three times as fast as a normal human child would, maturing in about seven years. To achieve the effect, a young actress was used for the motion capture and the as the framework for what Renesmee would look like at various stages of her development. CGI effects have come a long way, but it's still imperfect and can look unnatural.
Question: Since werewolves stop ageing, wouldn't humans notice?
Question: Will Renesmee ever die of old age? And would it be possible for her die from something a regular human could die from?
Question: What would Jacob have done if Renesmee left? Would he have followed her? Wouldn't it be emotionally painful for him to be away from her?
Question: Alice has a vision while Renesmee is hugging Bella and Edward and Jacob is watching. In Alice's vision, is the girl Jacob is with Renesmee all grown up? Are her and Jacob a thing since Jacob imprinted on her?
Answer: That is the grown-up Renesmee who Alice sees, but the vision is rather vague. As has been pointed out by the Cullens, Alice's visions are subjective. If the people she sees in them change direction or make different decisions, then the visions become different. It is implied that Jacob and Renesmee will become a romantic couple after she matures. Even if they don't, because Jacob has imprinted on her, he will always be her protector.
Answer: Aro also makes the argument that for the first time in our history humans pose a threat to our kind with their weapons that can destroy us. Theoretically any weapons that can tear apart and/or burn the vampires would work. So, yes, things other than vampires and werewolves can kill the vampires and werewolves.