It is possible to edit posts and comments on Hive. While original still will be in the blockchain, Apps will show the updated version. People use editing feature all the time.
Also it is unto App developers to build new to tools to make this possible. For example custom_json feature of Hive allows storing all kinds of text based data. One can develop an App that stores user data in an encrypted form on the database and users could choose who can view the data. When user decides to delete the data they will simply remove decrypting privileges.