A venomous snake does not, before it strikes, concern itself with the innocence of its prey. Mueller is a venomous snake, and if he had an honest bone in his body, he would have ended the investigation months ago. So Trump can not appear and expect an honest inquiry.
I have seen those lawyers on You Tube explain why you should NEVER talk to prosecutors in an interview. They can use your honest truth to convict you.
Mueller should never have accepted the job to begin with. He knew that he would have to investigate his friend Comey, and that for him to give Comey a pass would fail the "appearance of impropriety" test. He did not care. His anti-Trump agenda was all that mattered.