UX issue with questionnaire

We are developing a questionnaire for enterprise size company. Each staff member should periodically (annually, semi-annually) complete this questionnaire to reveal their compliance in various areas: Ethics, Financial Interests, Insurence etc.

The problem we are facing: The questionnaire consists of categories of questions. On the left-hand side, we have those. In the middle, there is a content area that displays the current question category (list of questions from a particular category). At the bottom, we have pinned navigation with following buttons: "Save", "Save and Continue", "Submit". Submit button technically is available ONLY when all the question categories are completed. But the designer decided to keep it anyway throughout all the steps. In my view, there's no need to display a button that is available only in the end when all the steps are completed.(questionnaire may take several days to complete)

I think we need "submit button" only when the user completes the steps. "Save" button for me also seems to be redundant since we have "save and continue" which is basically the next button. Also, do you guys think we need previous step button? I think we do, even though there is clickable nav pinned on the left (with the question categories)

Would appreciate if someone could provide related research

PS: Sorry about pic))