Check out our pricing page for more details.
But you also will need the Apple developer account ($99/year) or Google Play developer account ($25 once).
No, any changes made on your web page don't require an app to be rebuilt (Natively it's a simple web page wrapper).
Usually, Apple/Google's team describes the reason for rejections. If you are pretty sure it's related to the source code of a mobile app build, please get in touch with support and attach relevant screenshots. Otherwise, it can be related to your website, or some data you've filled out on the App Store Connect/Google Play app page is inappropriate. In such a case, there is no obligation on the Natively platform to issue you a rebuild or refund.
Yes, unfortunately, there is no other way to deploy a new native code to your existing mobile app.
You can try to run a new feature on your current build, if it's available, everything should work just fine, otherwise, you will get an error.
Unfortunately, Android WebView doesn't support iframe documents preview, you can try to wrap your document in a GVIEW: