What should one do if they fear the unlawfulness of obtaining grades in oral examinations through recommendation, yet at the same time fear academic loss due to some professors who might lower their grades significantly? Is this considered casting oneself into perdition?

Obtaining high academic grades through recommendations and favoritism from professors is a betrayal of education and forbidden deception. It is not permissible to do so, even if you think you will be unfairly graded, for falling into what is forbidden is the very essence of the destruction that is prohibited.