Is Reiki, as typically practiced, sacred healing work occurring through divine energy and what are its risks for allowing intrusion of dark spirit meddlers or extraterrestrial mind manipulation energy?

Unfortunately, as typically practiced, the situation is much the same as with Reconnective Healing. Without a partnership with Creator, and safety requested specifically, invoking energy, even "spiritual" energy is no guarantee, in and of itself. Many things are held sacred by people, including satanic worship. So, there must be some precision and specificity as to what is being invoked. Many view such modalities as a kind of borrowing of a tradition and assume it is real, that it will act as intended, and they can simply go through the motions, use the process as they are taught, and it will happen as described. That may well be the case, but all such processes, unless invoking the divine realm directly, will have limited reach and, in particular, will be risky because they are an invitation, in each and every case, and invitations are always answered. So, the same liabilities will apply that spirit possession and enhanced extraterrestrial psychic programming of the subconscious, of both practitioner and client, may be the end result.