See if I understand this correctly:

+A and B are in different fleets
+Both boats are on starboard
+B is heading to her downwind finish while A intends to round C-mark for another lap
+C mark is a gate and is placed upwind of the committee boat

Sounds to me like A has no rights here. B is sailing her proper course and the buoys at C are merely obstructions to her now. Both vessels are racing so racing rules apply, but both boats are not about to round the mark so the mark rounding rules do not. It also sounds like B's proper course was to maintain heading to the finish - especially since it sounds like she might have had trouble laying the finish line. Overlap doesn't really matter since mark rounding rules don't apply - No foul.

Granted, however, it would difficult for the boats to tell the intent of the other on the course.