@Bee 

If you have achieved the process of who sees which block, firstly that’s great!

Secondly, if you want to disable or not let the other person change or edit the already filled responses, you can use another embedded data along with a JavaScript code which you can define early in survey flow. 

So, let’s say, Mentor is looking at student’s responses so embedded data Mentor=1 you can pass in URL and use the below JavaScript in all the questions which are asked to a student.  Here I have disabled all Input elements except Next Button (as it would be required for navigation) if the mentor embedded data is 1.

Qualtrics.SurveyEngine.addOnload(function() {
    // Replace 'Mentor' with your embedded data variable name
    var MentorValue = "${e://Field/Mentor}";

    if (MentorValue === '1') {
        // Disable all input elements
        jQuery(':input:not(#NextButton)').prop('disabled', true);
    }
});

You can do similarly with Mentor and HOD combination and add that on both student and mentor questions.
Hope it helps!

Great! @Deepak  thanks a lot. It worked except that it doesn’t work on ‘signature’ questions. 

Question that allow respondents to sign are still changeable though. Can you assist with that please.

Try this

Qualtrics.SurveyEngine.addOnload(function() {
    // Replace 'Mentor' with your embedded data variable name
    var MentorValue = "${e://Field/Mentor}";

    if (MentorValue === '1') {
        // Disable all input elements
        jQuery(':input:not(#NextButton)').prop('disabled', true);
jQuery('canvas.SignatureCanvas').css('pointer-events', 'none');
    }
});

That worked @Deepak . You really are a PRO