Is it common that marker TA make the marking scheme instead of the professor? I had a course in which the instructor let the markers make the midterm marking scheme and had students ask the TAs to resolve any marking issues instead of talking to him, though the instructor provided sample answers. As far as I know, many TAs are inexperienced and I think leaving the job of making marking scheme to TAs may be too much for them. IMO, making marking scheme should be the instructor's duty. I don't know if it is common in university.
This might vary by country, but in the US, if the graders are undergraduates--which shouldn't be the case for midterms--then I would say that it's unusual to allow them to decide on the grading scheme. If the graders are grad students, it's quite common. At many schools even first-year grad students are the instructor of record for a course.