How much course preparation, grading, and student question handling can be reasonably expected of a graduate TA?
The professor has created no content for this course at all, and I put in at least 40 hours a week. Is the professor allowed to have me doing this much work?

It depends on the individual university, but in practice almost certainly not. I a math professor in the United States, and by that standard this sounds absurd. From what I can tell, the professor's behavior is completely out of line.
When I was a TA, I was responsible for: running discussion sections, holding three office hours a week, grading, and answering emails. And, if I felt nice (which I generally did), running review sessions before exams. That's it. I was not responsible for creating any of the content for the course.
I would recommend making an immediate appointment with the graduate director, and asking what the expectations of your TA role are. Remember that, as a graduate TA, the graduate director is your boss, not the professor for whom you teach. If your graduate director tells you that writing exams or preparing lecture notes is not within the scope of your duties, then you can and should ignore any requests from the professor to do it anyway.
Good luck to you!