The db: prefix must (according to the article) always be there. What he's describing is the URI part, which follows RFC 3986. It's not two formats, just one: db:<URI>.
Yes, but for convenience (and compatibility with other proposals), I think it would be okay for a given implementation to recognize well-known engines and treat them as valid DB URIs even in the absence of a <code>db:</code> prefix. I've added notes to the <a href="https://github.com/theory/uri-db">documentation</a> to that effect.
