How does the divine realm decide who to work on first, given a general request to heal humanity in increments, or is the divine reach powerful enough to commence work on everyone at once even though they are segmented into increments, tied to a measurement of the initial intention of the request, as done for each segment?

Indeed it is the latter, and this is logical and why you thought of it, that because the divine realm cannot play favorites, it must do all or none, so a request to do all humans must involve them receiving equal attention. So indeed, it is the case that all are divided into increments and then each increment has an initiated session but done for all practical purposes simultaneously, and so that will remain in force as the policy unless something different is requested.