I've seen cumulative grading (start at 0 and build from there) done in Canvas just by setting everything as a default of 0. Pretty straightforward to just do normal grading from that point.
Starting everyone off with a C (75% or whatever equivalent) seems more intricate... I think you'll run into issues using late/missing policies and default grades together, and it's going to look very confusing for students if you default everything to a grade, as they'll have a harder time knowing what is a default and what is real. In my head, I *think* you could end up making one no-submission assignment worth the total amount of points in the course to begin with, and set that to 75%. After grading each assignment, you'd go edit the no-submission assignment, reducing the amount of points it's worth by the assignment you just graded (so in the end, that ends up being 0 points and the students see their grade).
With that potential method in mind, I have to agree with @SusanNiemeyer that this grading method seems a little crazy to me, though I am a Canvas admin and not a teacher. I think the original teacher is trying to achieve a hybrid of regular and cumulative grading to get the best of both worlds, but I think it's likely to cause more confusion and work for everyone.
-Chris