It's really interesting to read about the homeschooling experience from the student side.
The description of your time with the first calc prof reminds me of my German A level class. I think there were only a couple of us who actually completed it in the end and barely passed, with an E.
I don't know how your letter grades work. At my college, an A, B, or C were passing grades in descending order. D grades overall in a class were not sufficient to proceed to the next level of coursework. An F was a complete failure.