The level-4 modifiers are actually useful - I don't see their inclusion as a failure of a standard.

Ambiguity was going to be possible either way - here's a Level 2 example:

    /{+a}/{+b}
            ->  "/foo/bar/baz"

By "reversible" I meant it's possible to find something that reconstructs to the original URL (http://bit.ly/1H27rwD), much like you did.