The fact the shortname isn't a valid manifest property? That's just from knowing the definition of the mini-manifest. The mini-manifest only needs to know:

- Name
- Package Path

At a minimum. Version, size, release_notes, developer, locales, and icons are optional, but should be specified optimally if known.

See https://developer.mozilla.org/en-US/docs/Web/Apps/Packaged_apps for more information.