Is it truly the case that people benefit from an optimal waiting period of nine days following their passing before carrying out a Spirit Rescue, or is that just the interval people who have no problem with transition might employ on their own to finish up earth plane business before going to the light?

The latter is the case and not the former. So you have nicely refined this perspective by appreciating that healing requests are all flexible, and can be adjusted as to timing as well by divine realm in their implementation. If a departing soul needs time close to earth plane, for example, to attend their funeral or see to distant relatives and be closer to them energetically to perhaps give them a sense of their presence as reassurance on hearing the news of their passing, the Spirit Rescue will not simply whisk them away and render it more difficult to attend to their affairs. If time in that way is needed, the divine light callers will simply wait patiently to see how the transition goes and when the human spirit is ready, and then if there is any difficulty will use the Protocol request to implement the full array of healing tools to prepare that spirit more thoroughly so that a successful transition will result. There is no necessity to wait. The rescue will always occur when permission is given from higher self, so if there were a reason for delay, that could be seen to and supported. The benefit of doing it early is logically apparent, particularly when spirits already on board the person's energy when they pass away will accompany them and begin, in many instances, to attack with renewed vigor, because now there is a smaller pool of energy available and there will be a serious fight among onboard spirit meddlers to remain in control of that individual in spirit form. So in the case of someone without spirit attachments, upon emerging from the body their spirit will also often be aggressively attacked by spirits desperately seeking sources of energy. So an arbitrary waiting period simply invites a period of torment to take place that could be avoided by doing the rescue quickly.