You don't need to download your app twice. Changes are done on the fly.
We are using an internal caching system. That is why some changes are taking some minutes until all users of your app will get it.
You can choose between diferrent styles (even more in the Pro-Plan). But if you want to have a more custom layout, you can create your own mobile page (e.g. nativeDroid) and use it as source here. So you're more flexible ;)
You can use a website (WebView element) where users can upload files. This function is activated in our apps.
Attention: Google removed this function partially in subversions of Android 4.4 (Info). But this problem is resolved in later Android versions. So the most users should be able to upload files in your app.