204 is the code for “this thing is gone” so passing a user ID that does not exist will just confirm that it’s not there. That code will tell you that the user ID is gone, regardless of what you send it. If you want to check for the existence of a user before that, you’ll want to get the user first:
Corresponding method:
For your second question … the model we generally use is that the user data is under control of the tenant/account that manages the user store. Users can update their own passwords (reset password link) but, aside from that, your app will need to decide what a user can and cannot do and implement that via the Management API. Using the session in your app to confirm that an action can be taken, you can implement a profile edit screen and do a client credentials grant to perform the changes:
As of v4.6.0, that will be performed automatically if you don’t pass in an API token.