What's the Deal with the Original Negatives (Shout Remaster)?

How come Shout is using an interpositive to restore the movie instead of the original camera negative? I am a little concerned - not about the upcoming Blu-ray, but about the survival status of the original film elements. I heard the restoration team only recently found original audio elements (which they said will be included on the upcoming disc to provide lossless audio), but did somebody throw out/destroy the original negatives? I don't mean to complain (I am eagerly awaiting the new release), but the possibility that someone just junked this material is disturbing (especially when you consider how much of film history has been lost forever).

Also, does anyone know if the voice actors credited for characters who did not appear/speak in the finished product actually recorded any dialogue? I know the VA for Inferno was credited for the character, even though Inferno did not have any lines in the final release? Would be interesting to see if any unused finished animation or unused dialogue still exists (aside from what has been released on any of the DVDs or the original trailer/demo [concept?] reel).