I don't doubt that his struggling to get a C- in calculus reflects some sort of relative lack ability on his part, but I don't think that it carves reality at its joints to call that "mathematical ability."

Sounds more like a lack of enthusiasm. Allow me to illustrate. There's a story of Thomas Hobbes finding a copy of Euclid's Elements on the table at a friend's house. He opened it up, found a proposition, and disbelieved it at first. Then he started reading the proof. Whenever a previous result was referenced, he looked up that proposition, went over its proof, and so on. Eventually, he made his way back to the beginning of the book and became amazed at the whole structure--of a seemingly far out result being carefully built on an edifice founded on statements so obvious that no one could dispute. He gained a great deal of respect for geometry and you can see some echo of this sort of thinking in his Leviathan.

Without that kind of spark (and the discipline to back it up), study just becomes an exercise in drudgery. If your motivation to learn a subject is to get an A so you can go to law school and please your father, then class performance turns into a game of Guessing the Teacher's Password. Some have a lot of trouble forcing themselves to play.

Separately, I think that his calculus experience would have been very different if it had been immersive: I don't think that he would have gotten a grade below C in calculus if he had spent all waking hours talking about calculus with me for 6 months.

I certainly hope so. I very much doubt the average C student spends more than 10 hours a week (including classroom time) for one semester doing calculus problems. Retreating to a monastery and meditating upon the calculus for six months should work for any student.

Scott might just have the problem where he has trouble proceeding to a new step without understanding the old ones, and the class went too fast to keep up. Some students can manage by taking everything on faith or mimicking what the professor did on the blackboard, but this causes other students much distress.

As far as I can tell, it's empirically true that Scott's emotional reaction to the unsolvability of quintic is unusual amongst mathematicians (while being almost uniform amongst elite mathematicians).

Unsolvability? Bah. It just takes a radical approach (well, figuratively, not actually using radicals).