A question regarding vessel forms!

If a shinki takes the form of a non-traditional weapon, would we be able to take liberties with what it can do, for combat purposes?

For example, this guy's pretty silly, so I was thinking about having him shift into an umbrella that could snap closed to "chew up" ayakashi. Would it also be safe to assume that the weirder could block attacks with it? Could I go super crazy and have stuff shoot out of the top cap, too?

Or would that have to fall under the shinki power?