Got a C in a sophomore subject unrelated to my desired major... how does this affect Ivy/Oxbridge chances?

When it’s unrelated to the major, adcoms can ignore it (or place less value on it.) Much more critical would be appplying to US tippy tops with an overall B average. In your hs, is that 91 a solid A or the low end?

Can your GC explain something reasonable about why you got a zero on the final? That can help. A short mention that uses the oportunity to talk about your great attributes despite this.

Whether the retake situation shows is a matter of your hs policy. Some hs will report the original class, as well as the repeat. I agree, you have better things you could be doing with that time than re-taking an elective.