Question

Can you use one my-release-key.keystore to sign different apps or do you need a different my-release-key.keystore for each app?

Answer 1

This is lifted straight from the Android documentation – bolded the important bit.

Some aspects of application signing may affect how you approach the
development of your application, especially if you are planning to
release multiple applications.

In general, the recommended strategy for all developers is to sign all
of your applications with the same certificate, throughout the
expected lifespan of your applications. There are several reasons why
you should do so:

• Application upgrade – As you release updates to your application, you
  will want to continue to sign the updates with the same certificate or
  set of certificates, if you want users to upgrade seamlessly to the
  new version. When the system is installing an update to an
  application, it compares the certificate(s) in the new version with
  those in the existing version. If the certificates match exactly,
  including both the certificate data and order, then the system allows
  the update. If you sign the new version without using matching
  certificates, you will also need to assign a different package name to
  the application — in this case, the user installs the new version as a
  completely new application.
• Application modularity – The Android
  system allows applications that are signed by the same certificate to
  run in the same process, if the applications so requests, so that the
  system treats them as a single application. In this way you can deploy
  your application in modules, and users can update each of the modules
  independently if needed.
• Code/data sharing through permissions – The
  Android system provides signature-based permissions enforcement, so
  that an application can expose functionality to another application
  that is signed with a specified certificate. By signing multiple
  applications with the same certificate and using signature-based
  permissions checks, your applications can share code and data in a
  secure manner.

Documentation