As you are integrating Push Notification in your project. Here are the necessary steps you can follow.
Add Configuration details in your AndroidManifest.xml file with the help of sample Manifest.
As your project already have google play service, in this case don't add google play service available in sample project.
Yes, you will be able to see registered users in Users section of Unified Notification Panel. You will be getting errorCode=NotRegistered, when application is Uninstalled on the device.
As you managed to work with the Sample then the Token can belongs o that sample.
Yes, Device token can be different from same device.
Have you able to register on App42 with your existing project. If not, then might be AndroidManifest.xml is not properly configured.
Let me know the details.