There are form load rules in the Data tab which you can use to switch views when the form is opened. Just create a rule to check if status = rejected then switch to your read-only view. This will prevent users from seeing the editable section. Don't forget to disable the views dropdown in the ribbon as well (File -> Form Options -> Web Browser Category -> Uncheck Views).
If you know who the supervisor is, you can use the userName() function to determine the current user and perform different logic based on who is viewing the form and perhaps what state the form is as well.
Sounds like a pretty complicated form for your first InfoPath experience :)