Who or what was The Buddha, his mission on Earth, and how can Buddhism be reconciled with these channeled messages of the very existence of a Creator, let alone the encouragement of human partnership with Creator, when the Pali scriptures say the Buddha was only concerned about one primary matter, which was/is the complete cessation of freedom from suffering (dukkha) or the unshakeable freedom of mind, and that ultimately the Buddha rejected both theism and the soul theory because they are delusions and objects of attachment?

This answer contains some discomfiting material and we like to apologize in advance for disturbing cherished notions held by many, but when we are approached with a sincere desire to understand seeming contradictions we must respond in kind with truthfulness and a straightforward accounting of the explanation why this may be so. The Buddha was an advanced being because of advanced awareness of the divine. This is not turning reality on its head as one might think, given that preamble within your question that he represented a summary rejection of the idea of a divine, supreme source. Everyone has some intuitive reach. What that is, represents a divine level of engagement with the broader universe, courtesy of the apparatus within the human being to use the subconscious gateway to engage the non-local consciousness. That is an aspect of the soul and its workings, projected into human beings as their overall composite awareness, but with the special qualities that the deep subconscious that is organized by a special level of energy, includes the non-local capability of existing everywhere at once and enabling one to connect with the broad array of targets for purposes of learning and potential influence and interaction, in an iterative fashion, as with having a conversation. And that, indeed, is how this dialog is being conducted. It is through the same means. So Buddha was picking up many messages with knowings and understanding of metaphysical phenomena and capabilities, but in a subtle way, coming in largely through his higher-self, his soul intermediary that is the blessed part of each being acting as a custodian, as a helper and a monitor as well, of all that happens for someone within the physical plane, but represents the divine level and follows rules of the divine in all that it does. Buddha, as well, received impulsed information directly from Creator. All of these impulsed ideas, suggestions, reminders, and messages of encouragement, at times, acting as a reward to keep going with a good thought, to keep pursuing an idea because it has value left for him to decide, personally, through his free-will what to do or not to do with his time and focused concentration. That awareness allowed him to cultivate many deep understandings about life and was very much in divine alignment with regard to the importance of the lofty idea—the truth of divine existence and hopefully what that might mean to the individual. He embraced this awareness quite readily but being a pragmatic person also, and following his intuition, when it failed to show him the face of God, the embodiment of the divine, he was not willing to proceed too much further on faith alone, even though many before him embraced those ideas simply because they were open to doing so and were not very self-reflecting in the potential implications. So, someone who is of a practical orientation wanting proof, wanting validation, can in many instances limit their growth by requesting that the divine make the first move. This can be a fatal error for the metaphysical and spiritual explorer, because this violates the rules of engagement decided on for the human enterprise: that humans are in charge of their experience. They are in charge of their learning and growth. They are in charge of their very lives, and the divine can only follow their lead. So, anytime someone establishes a demand for proof, a demonstration that an idea, or notion, or capability that might be possible be demonstrated to the liking of the questioner, this will very likely not happen because the divine cannot lead you to faith. This is why it is a test, to see what you can believe in your heart of hearts and expand on, in fact, to embrace the divine with ever-increasing fervor, trust, and acceptance. This is demanding. It asks a lot of each person because of the level of disconnection humans suffer. Humans have been robbed of their antenna, so to speak. So, you are not receiving divine words and messages and images in a coherent, direct, demonstrable way within your being. It is coming in faintly and bubbling up from deep below as though it is your idea or random thoughts. It doesn't come with a flare, with a flourish, with angelic chorus accompaniment and blazing light across the heavens, thunder and lightning for emphasis. It is just a thought that floats up, even though it may come from God directly. Given that constraint, it is quite a challenge for people to move much beyond thinking about what they themselves can do and control. The Buddha knew there was something beyond the self. That was because of his non-local consciousness bringing a stream of knowledge to him in a way he knew came from beyond. What largely happened here, to make a long story short, and to avoid pointing fingers and naming names, was the manipulation of his teachings. Rather than extend them into wider currency and greater applicability as a starting point that could continue to grow and add new revelations, increased awarenesses and a wider perception of the existence of many layers of divine intelligence available to integrate and interact with humanity, his acolytes adopted the more stringent and limited perspective of his doubts and ended up changing his message in a way that it would constrain belief in the divine. And inexplicably, to the outside observer, if you believe what we have been saying, this added extra constraints on an already difficult situation suffering from constraints in the first place. Those constraints were imposed by interlopers. His acolytes were influenced by interlopers, as well, to constrain their thinking and to limit their ability to see beyond what Buddha had described, and to tighten the noose even tighter on what thinking was allowed. The end result of this was a greater disconnection than Buddha himself struggled with. Each human being undergoes the same progression as Buddha. Many stop on day one and go no further than just knowing they are alive, and live a prosaic existence with rules handed to them by others that are unquestioned, and there is a profound lack of creativity exhibited ever. This, correspondingly, means there will be no growth and no advancement of the soul journey. In a sense, it is a token lifetime and will often end up with the individual leaving the scene with many impairments from the inevitable slings and arrows of life, and will have a storehouse of unmet healing need added to, going forward. This eventually will work out and to the person's advantage but is certainly not ideal, and falls far below what is possible. So, we would say that the pursuit of the ascetic ideal of disconnection to insulate oneself and be separate from material perception and the imperatives of the physical existence is a denial of reality that you are, for the time being, a physical human with physical needs and demands. It is, in a sense, cutting off a portion of human experience and making the decision up front to simply not engage. It is like being born and deciding that walking about is futile and purposeless and deciding to leave the legs unused and to wither. The life will inevitably be constrained in many, many, many ways as a consequence. This is no less true when one gives up the inner yearning of the soul to reconnect with its divine origin through the physical human, to close that loop, and make an energetic infusion of awareness, insight and divine love as fuel. That can catapult people to greatness and, conversely, its absence will lead to a hollow existence in the end.