Agreed, it's terrible. It's amazing there is no plugin for Gradle that does it all for you, including splitting the app for different dpi and arch. Is there a way and it just isn't publicized? AFAIK Google Play does have an API..
Well, they should just be doing it for you on the back-end. In fact, all you should have to do is upload an APK with all high-res assets and google should downsample them as appropriate for the downloading device. But they don't so...
Well, your signing of the package would just be a signature to Google that it was indeed you who uploaded it. Google would then sign the new packages as themselves and the Play store could only download and install Google signed packages.
Android package security (who can send intents to who) is based off of signatures and it's not really possible or desirable to change that.
However, a build script to handle it all for you during upload would be a great way to solve it.
The API for this seems to lack the features that you would need though [1]. It doesn't appear to let you upload apps for only certain devices - unless you would specify it in the manifest of the app somehow?
Also amusing - the API says "Do not supply a request body with this method", yet obviously you'd have to supply a request body - the app itself! I have a feeling this is not a well-used API.
That's probably the understatement of the day. The Play developer console is such a pain in the ass to work with that I'd rather not.