FD, Re your point B),
This assumes that an 'acceptable' point of touchdown wrt the markings will provide sufficient runway remaining in which to stop.
Whilst in most normal operations the remaining distance is sufficient, the operation is still based on an assumption which might not hold in all circumstances because of the uncertainty about the remaining ground roll distance - depends where you touchdown. i.e you can land within a generalised zone (TDZ markings), but still be wrong.
Furthermore, 'inherent margins' are generally unknown, with wide variations according to conditions, accuracy of reporting, how the landing is flown - slightly high, slightly fast - most of the margin already used.
LDA constant; LDR variable.
We are slaves of our perception