With regards to the Questions:
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To do healing with VK one doesn’t need any initiation as you are not the Healer, VK is (I am sure as a VK user you may have already experienced it). Even if you are sending the energy of Reiki, without prior initiation, VK will send the energy of the highest level (in this case that of Grandmaster). Now with regards to the 2nd part of the question, whether a Guru is needed or not, then the answer to the same is Yes although not for spiritual evolvement, but for constant guidance. For ex. under Hatha Yoga, the seeker practices for years together to prepare oneself for the ultimate form of Yoga i.e. Sadhana. If we try to enter in Sadhana without prior practice or understanding(initiation) then it may result in adverse effects. Another ex. based on your question of Reiki Attunement – If one is not initiated or guided then one may make the mistake of trying to attune themselves with either all the levels in a single day or in a single instance or directly try to attune themselves with the Grandmaster level attunement. As a Reiki Grandmaster I can assure you that it shall result in severe problems either at a mental or emotional level as those energies are too much for the self to accept in one go. Thus, a Guru is required to gauge one’s progress and then guide accordingly.
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When the spiritual masters quote that “powers” (whether through sadhana or otherwise) may act as a hindrance to one’s spiritual progress, what they are trying to say here is that power corrupts. So if one gets hooked on to a certain power/siddhi and doesn’t focus or continues the spiritual journey/sadhana then yes it definitely is a hinderance. But they also say, that siddhis are always a by-product of constant spiritual sadhana, but one must not focus or get stuck with the same, rather they must continue their spiritual journey. This is another reason why a Guru is needed because they can identify when the student slips and then guide them back onto the path. Also as per the scriptures, there is only One Truth (Brahma Satyam – Jagat Mithya), so there is cannot be my truth or your truth. So yes if you keep requesting VK to give you energy for spiritual growth VK shall bless you with the same, but you shall have to be extremely alert to know when you slip. Since you happen to mention Adi Shankaracharyajis Atma Shatakam I’ll recommend that you go through Sadhana Panchakam written by him, where he clearly mentions the four-fold qualifications of a seeker, also known as Sadhana Chatushtaya. This shall help you gain a much better understanding on the above concept.
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How VK helps in fast tracking spiritual growth is by providing one with infinite/abundant energy. But it’s upto the person to constantly use VK to keep raising his/her energy frequency. Also what @"Sanjay Roy-Certified VK Teacher" Sir was trying to convey here is that VK can definitely be used for Spiritual Awakening (is already being used by many), but one has to physically take the effort to sit in Sadhana everyday until the goal is achieved (time may depend from person to person depending or their own energy frequency). With regards to the second part of the question, from what I remember, Hira Ratan Manekji (HRM), had mentioned that it took him years of sadhana and experimenting until he perfected the method of sun eating (10 secs per day etc.) and even after he had achieved his intended goal, he continued sun eating and did not stop. Similarly, even spiritual masters who’s Kundalini is fully awakened and they have realized their true self continue their sadhana till their last day, simply because they are aware that this human self itself is not permanent. So to put it in other words, how can a temporary container contain the infinite permanently? It can only be merely experienced until one truly becomes one with the infinite. Even in Bhagawat Geeta Lord Krishna whom we know as a poorna avatar and had access to all the 16 cosmic rays (VK is attuned with 11 cosmic rays) says the same and emphasises on a daily sadhana as long as one is in the manushya (human) yoni. The same is also mentioned in Bhagwatam and other parts of Mahabharata. It will also interest you to know that all these spiritual masters had a Guru, Adi Shankaracharyajis Guru were Shri Govind Bhagvatpadaji and even Shri Krishna’s Guru were Maharishi Sandipaniji.
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VK does operate at a much higher dimension/frequency, but the reason the results are temporary for us is because we ourselves are temporary. So, if one contemplates from a scientific perspective, what truly is permanent? Only Energy is… To become permanent, I urge you to constantly keep practicing and rising in your Sadhana.
Thus, to answer the core question, yes through VK one can receive attunement and as pointed out by you it’s mentioned in the manual as well. But as @"Sanjay Roy-Certified VK Teacher" Sir has emphasised it is best to be done under the guidance of a Guru. You may also refer Guru Ashtakam by Adi Shankaracharya ji to understand the significance of a Living Guru.
So Thank you very much Dear @Sajiv for this Beautiful Satsang and keep questioning, for a question is a key to an answer. Hope I was able to quench your doubts for the time being. 
<3