In my experience, NotFound can be returned when you do not have permission to access the object in the Exchange store. Make sure that whatever credentials you use had full mailbox access rights to the target MB. (I suppose if all you want to do is read items, you might need fewer permissions, but you can trim later on if needed.) I don't see you setting any credentials in the above, so presumably you are accessing EWS with your default creds, i.e. what you're logged in as when running this.
And no, you cannot get a WebDAV response stream from EWS: they are totally different services, and as I'm sure you're aware, WebDAV does not exist after E2007.