1) Inuyasha is linked to Kagome in a way that no one else is. Plus, as we wind up seeing, Kagome controls the well through her sheer force of will...So she's probably the one that let Inuyasha in only.
2) The anime stops halfway through because-like a *lot* of anime out there-it didn't have the time, money or material to keep going. (material meaning that it had caught up to the manga's current story arc and Takahashi had to produce more to keep it up.) There are a BUNCH of anime that stop in the middle of a story. OHHC, Fruits Basket, Rurouni Kenshin (TV). etc. Though once the manga was completed in June 2008, there was still so much of a fan-base that the producers as Sunrise figured they might as well finish the anime off too. But-they didn't want to spend a lot of time, money or resources on it even so-so we wound up with a rushed, timeline-fuqed, smooshed together 26 episode run that zipped through 20 volumes of the rest of the manga.
3) That's a mixed bag there. There are some points where its very well done, and then other points where its not well done at all, and doesn't say even remotely what was said. I'll blame US censorship for that though.
4) No. Naraku was killing people before Voldemort or Sephiroth were a blink in anyone's eye. Both Harry Potter and Final Fantasy VII were put out in 1997, respectively. Inuyasha began its serialization a year before in 1996.
5) Viz didn't. Viz is the American company that takes over translation and dubbing for anime and manga coming to the US. The people who originally serialized the Inuyasha manga did it actually. Shonen Sunday Comics. And they did it for the cash cow that Inuyasha turned into. They were making more money with them.
6) The ani-manga only went as far as far as episode 88: "The Monkey God's Three Spirits."
7) It's not just you. Pretty much everyone that matters' father is dead. There are a few exceptions... Kanta, the otter youkai, and Shinta, the boy with whom Kohaku-kun stayed with just before meeting up with Kikyou. But-no one important has a father.
Hope these answered your questions! Thanks for them!