I think the main point of the whole exercise is simply to give elders the smug feeling that they've done something extraordinarily wonderful by understanding and applying some (pretty arbitrary) rules correctly.
You can see it all over the middle elder's face, that he really thinks they have scaled the heights of pure wisdom itself by ingeniously deciphering the "correct" solution to each case. They consider it a blessing for the young sister to be relieved of pioneering, plus they feel a warm glow that the decision "upholds the high standards of pioneering", that the rules about the elder taking time off are wise and just, that showing consideration for the older sister is a demonstration of unparalleled compassion, but that in all cases it's absolutely vital to understand and follow the minutiae of the instructions and always consult the circuit overseer when indicated.
They could just as easily have come to completely the opposite solutions: remove elder and old sister as pioneers, and keep younger sister, quote scriptures and make up rules to justify each decision. Like many Watchtower rules they are arbitrary and subject to change.
But having such rules fosters the idea that they belong to a considerate and orderly theocratic system, and they derive their own sense of worth by recognising and respecting the instructions and their own role in implementing them.