What is the ruling on a student taking grades he does not deserve due to "wāsiṭah" (mediation/favoritism), after being wronged in previous grades?

There is no issue with the permissibility of your intercession with the professor to be fair to you and give you the grades you deserve. And if the professor gives you more than you deserve, there is nothing upon you, and the professor will be held accountable for what he did. You should also disregard obsessive thoughts.