Google updates all the time. If your site is down all the time then you are screwed. If its just down for a day or two then google will come back.
Actually from what I understand , google uses a certain snapshot to decide the PR thingy ... I've read someone here had a site down at the time of that specific calculation , and because of that , his page's PR just got completely removed . I might be wrong though .
You positive ? Even if google tries and fails to crawl the url that was linked to ? I was under the impression that there is also the issue of the relevance of the anchor text to the page's content , so since there is no page content or description , it would be 0 . Or is accuracy a whole different thing ? I'm newbie , so if you can really elaborate it'd really help me . I was sure that accuracy is the relevancy of the links to page's content .
You are right, Links and backlinks must be on similar subjects to get more weight on them. What mad4 meant was if a site has few backlinks with a page in it, still the site will have a PR, but ranking well in SERP is another issue. I hope it clarifies your ? ~Imran