1. It could be a Google error. Google frequently screws up. I've occasionally seen pages that have been gone for over a year that Webmaster Tools reports as 404 errors, even though there aren't any links anywhere to the obsolete URLs. The obsolete URLs are generating a proper 404 status code, which means that G should have removed them from their index shortly after the pages were removed.
2. If the sitemap is in the root directory (where it should be) you should not have to tell GoogleBot where to find it, but you can try adding the sitemap path directive to the robots.txt file.
Everything in the robots.txt file is case sensitive, so make sure that this is correct.
Google, Yahoo and MSN all use this.