1.) What Selected User are you talking about? The email notifications in the JIRA Workflow transitions are defined in that transition's post-function. By default, there are pre-defined "events" that fires off pre-defined email templates for you (Issue Created event which will send a issue created email). You can add the selected user to the right role as per your project's notification scheme. Alternatively, use the "Send Custom Email" (ScriptRunner) that can send custom made emails to whatever email you like.
2.) How do you mean by block? If you don't want some custom fields to be edited upon transition, just remove them in that transition screen.
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