How does a B- look on a transcript for a student looking to apply to top-tier schools?

Answered

No, one B- is not going to screw you up for top schools. You don't need perfection for these schools (and conversely, perfection guarantees nothing either); you just need your academics to be good enough to cross whatever academic threshold those schools have for students of your background. At that point, your ECs and essays will become much more important. We usually talk about that in cumulative unweighted GPA, and I'd say as long as your overall GPA ends up around a 3.6 or above, you should be fine (and you might even be okay a bit below that). You shouldn't have any problems hitting that mark if you're getting A+s in all your other courses.

As a sidenote, if you're not qualified, you would not be deferred. Deferrals are not soft rejections; they happen when a college gets many qualified applicants and wants to compare those applicants against their RD applicants before deciding. That means that only qualified applicants that a college would truly consider admitting will get deferred—otherwise they would not put themselves through reading those applications a second time. Same for waitlists. Applicants that do not cross the academic thresholds are typically rejected almost automatically, but those thresholds are lower than people think.

Beyond that, you sound incredibly accomplished, and I think the thing that will help your application stand out the most is figuring out how your many activities and perhaps your overall academic goals fit together. Typically, successful applicants to these schools will demonstrate a cohesiveness through their main activities and (possibly but not always) what they want to study; usually, that cohesiveness really comes through in essays, and in your writing on the application itself. If you can figure out those links and lay out a compelling picture of yourself, that's the most powerful thing you can do.

So in short, don't sweat the B-. It happens. Just try to keep your unweighted GPA above a 3.6~3.7 or so. Moreover, I'd prepare for that kind of thing in the future—grading is usually harsher in college than it is in high school, and professors rarely ever give extra credit opportunities upon request. Small failures are a good thing to get used to and be able to roll with, because no one ever has success every step of the way.