Send notification to all developers when a release fails in VSTS (prev. VSO)
You can only send the email alert to "Environment owner" & "Release creator" and there isn't any way to customize the content in the email alert for now, you can submit a feature request on VSTS User Voice.
Rest API is a good tool to get these information. But you mentioned that you are using the same release definition for all branches. This does not make sense. Different branches refer to different version of your project, why do you deploy all of them in one release pipeline? If you configure your workflow to one release definition per branch, the resolution would be more easy. You just need to get the Build ID for previous release, and then get the changes after that build. And you can also add a powershell task to create a lable/tag for TFVC/Git repositories for current release version so that you can get the changes between the labels/tags.