Assuming we cannot request the entirety of the collective unconscious be healed of negativity, what is the best way to proceed in requesting cleanup?

The key is to tie it to the individual client and their dilemmas. You need not heal the entire history of humanity to help them, but only those things that are rumbling and holding them back in their current life. So healing the set of issues they carry and the energetic underpinnings, and those dark times in their own conscious awareness reinforced with their own negative thinking, and also drawing from the well a set of resonating dark emotions from many others who have experienced a similar dilemma and added to the collective unconscious pool, will go a long way to set things right and allow achieving an inner stability once again, and will let them stand down when they no longer have to wrestle these problems ganging up on them. So to have healing done for those issues pertaining to the client's dilemmas, the chief problems they face in their desire for harmony and peace, and well-being on all levels, physically, mentally, emotionally, and spiritually, will allow the divine realm wide latitude to go into the akashic records as deeply as needed. To clear out the thought plane of dark imagery, and then to remove a corresponding set of issues they may have contributed themselves to the collective unconscious that would then act as magnets to draw the collective experiencing of what they suffer as a kind of reinforcement and encouragement that they are very much on the right track and justified to be worrying about these things, to clean out that connection to the thought of the collective will serve to insulate them. It is much like having many places to dock one's boat and choosing a dock where the lights are on, and people are happily strolling about, having fun and being carefree, versus a dock that is cast in shadows, where there are lurking many unknown hazards. One can make a choice. So if one approaches the collective unconscious looking for confirmation of the dark notion, of a suspicion, confirmation for a dark belief being trotted out, one will find it somewhere. So the less one is negative, the less the response will be, with respect to intensity and validation, for that query. If the healing gets there first and serves to calm the waters and shine some light, a tentative query done with uncertainty will be less likely to bring forth a torrent of negativity to pile on, and open the wounds, and cause further discord.