If healing cannot be commenced for everyone at once, if a practitioner has a desire to prioritize healing for certain segments of society, can that be specified, and will those specifications be acted on by divine realm?

There can be requests to give special consideration to particular groups of human beings, but when this is done as part of a general request for all, there will be no practical difference because all of the healing will be launched for all practical purposes at the same time. There will be a slight benefit because of the requested preference, but people can choose to only do those segments of the population they care most about and leave requests for others to the work of other practitioners. It would be unfortunate though, for them to do so when they can make a simple request to help so many others at no cost to themselves.