Make sure you apply to UPitt early. You are a shoe-in if you apply by September, but if you look at this year’s CC thread, CS filled up fast and Pitt waitlisted candidates with 1550ish SATs who applied later. It seems crazy, but there are limited seats in that program and once it is filled, it’s filled.
A B usually isn’t a big deal, but because you go to an “uncompetitive” school and the B was in a class in your field of interest, tippy top schools may wonder if you are prepared for the rigor, especially if you apply TO. Your application is spectacular, and as you said you have already had success with elite scholarships and summer programs so you clearly can present yourself well. I might consider having your guidance counselor mention the time conflict with the DE class. It probably won’t be an issue, but the combination of a high school that might not be considered rigorous and it being in your major does worry me a little. An AO would need to be looking for a reason to exclude you, though.