For one thing, mysticism isn't a science but the study of mysticism is because it attempts to collect evidence, resort to empirical studies, define and categorize. It's thus as much a strand of science as psychology is. As for the experiences of mystical nature, yes they are basically either of a dualist or oneness nature, however "Oneness" is generally considered a more desirable AND higher state of consciousness on the ladder of consciousness expansion. Whether these are true or not is of course impossible to determine for sure as they can only be of a subjective nature but there are some contemporary writers who have done a good job in defining these things.
It may also be useful to think of mysticism as esoteric as opposed to exoteric. Esoteric is the practice of spirituality/religion that looks inwards for the answers and experiences, whereas exoteric is the opposite. Most traditional forms of relgion are of course exoteric as they rely a lot on a dualistic approach to some form of God or Gods and also rituals such as various forms of sacrifice. In short I don't see there is much one can argue about when it comes to mysticism!
This reminds me of the Taoist idea that you cannot speak of that which cannot be spoken of. As soon as you try, it no longer is the ineffable. The whole point with a real mystical experience seems to me to be to transcend words, since words rely on a dualist experience of reality. The experience of the underlying oneness of all things is the "goal" or point. On another note, not that many people have really been there and if they have, it's usually been really shortlived. What the future hold is another story though. Who knows what will become accessible to consciousness when evolution (the evolution of conciousness in this case) takes us further?
There are apparently many ways of conceiving dualism but I'm referring to the idea of yin and yang. In my "simple" world there is high and low, light and dark, happy and sad, etc... classic examples of the play of opposites in a dualist world. We tend to choose sides, take a stand for one thing and shun its opposites. I find this working fairly well in my day-to-day experiences and attempts to understand how humans act, for instance. Understanding life as a paradox where you attempt to join these together in a both-and kind of thinking rather than the adverse either-or. It's not, according the Buddhist philospher Nagarjuna, the ultimate answer to the ineffable, but closer to the understanding and hopefully consequent experience of oneness. A real life-changning mystical experience would of course be something akin to enlightenment, but it doesn't (as far as I understand) guarantee that life be free of conflicts caused by life in a dualist world.
However, even if things are not complete extremes but rather somewhere on a gray scale, it doesn't mean they would not be situated opposite of each other. In terms of psychology, which I know best, it seems to me that people do drift into extremes before they "come to" and realize that change has to be made. Or then change just happens, I don't know but I actually think we are less in charge than we believe. This would be a bit like the Hegelian idea of thesis-antithesis=synthesis. This kind of dialectic process would eventually lead to a higher state of consciuosness where the sense of self would be dissolved without getting lost. The sense of self gets lost if you are mentally ill and lose yourself in psychosis, for instance. I'm not a fan of Western thinking so I personally prefer the Buddhist idea that the sense of self is ultimately an illusion and so one can actually have an experience in which the illusion disappears from consciousness. There are some researchers of consciousness who promptly deny this sort of possibility but many who are acquainted with the Asian way of looking at it don't see it as impossible. My own experience of meditation is that you have an object to focus on for a while (it could actually be music, it works really well) and that focus then brings about a more expansive and profound experience of one's own being. I suspect that's a preliminary to a mystical experience.
Take a cat (because I have three of them): are they thinking? In my opinion they are pretty highly evolved beings and decision makers, but they don't have language. So they have I suppose all these images and impressions in their minds, and they are also able to remember things for various lengths of time. There's a lot going on there but they are still way more present in the now than we are, because we rely on language to entertain us about the past and the future. Perhaps a cat already is a mystic? They certainly have no trouble feeling connected to life in every way (unless of course their psyche's been badly damaged). Or can only humans who are able to transcend language, be potential mystics? These are questions that are surely almost impossible to answer.I wouldn't say Buddhists are agnostics. They have a hell of a lot of beliefs about other dimensions... No one ever claimed there is no God, only that it's ineffable and so in my opinion they are careful about defining it.
As for the Supreme Consciousness, well Oneness implies that it's everywhere. In a sense it's "underlying" all of creation and pervading it yet that's just a way of trying to explain it semantically that obviously doesn't catch the "overall" character of that Supreme "something". Here is where paradox would again enter the scene because Oneness is One AND duality at the same time. It's just us who cannot conceive of such great things who call these things such and such... whereas I'm sure that from some more enlightened point of view there is no problem. Why I'm sure? Uh, just what I believe :-). I do think the inner knowing about the state of affairs that is not tied to language is possible but it does require language to step aside. That's the difference between traditional prayer and meditation; in prayer you talk a lot! In meditation you attempt to bypass the alluring nature of blahblahblah that ties you to the timeline. Time, of course, being yet another illusion that we already know is not a "palpable thing" but a phenomenon relative to objects in space: time seems to take us from a to b but in fact, there are incidents where time is not an issue regarding our experience of reality. One case is that of psychological experiences that may be as vivid now as they were 30 years ago. It seems to me that there is yet another fundamental paradox related to the fact that time is an illusion and everything is available in the now, and our experience of evolution and a timeline. There are in fact many things we already take for granted as part of our day-to-day lives yet we find hard to explain. I personally don't understand how I can talk from one mobile phone to another, lol! Why would the idea of various states of consciousness meet with such resistance then? As you say, even coma is a state of consciousness. They say the person in question may even be aware of what is going on. Ah, so little we know yet!
Artwork: "Between Heaven and Earth", artwork by author, all rights reserved 1998