I will look around for some of these theologians and see if I can manage to read them. I find a lot of the philosophical work heavy going.
I also have trouble with the concept of God "participating" in human life in the way you describe. It presupposes a God external to human life, and that is a slippery slope. At this point, I could say with the Quakers that there is something of God in each of us, but that something does not have an external presence except as individuals share their "somethings" with one another.
I can agree with you and Spong that there must have been something astounding about Jesus for people to have retained a commitment to him after his execution. Finding ways to describe that without reference to the supernatural eluded them, but it doesn't have to elude us.
Jesus was clearly concerned with the ethic of human life as being lived around him. He (or his followers) described that in religious terms, but it seems to me the emphasis in Jesus' teaching is not belief in God (regardless of how that is defined), but in right action here and now. It seems just as valid to view his use of God imagery as a way of validating his moral and ethical stance, rather than viewing the morality as flowing from God.
Much of our discussion here is about language, or perhaps semiotics. How we describe our internal process is not as important as how that internal process shapes our external life. To use traditional language, "by their fruits shall you know them".
If we lived in a different part of the world, we might look for inspiration to Mohammed or the Buddha or Kahili ("the ring, he's wearing the ring"). Jesus is a powerful image to us because of our cultural tradition but the mainstream of that tradition has worked feverishly for 2 thousand years to make his image conform to their understanding of power, rather than his. Unless we can recapture some of that original power (not an external force in my book), Jesus is not useful to us.
The first thing I would say, specifically in regards to the "heavy-hitting" theologians, only Tillich is really so immersed in the kind of dense German bad writing that he can be off-putting. One of Bonhoeffer's many virtues is the clarity of his writing and exposition. Richard Rubenstein is all-American, meaning he has not been translated from the angular Teutonic with its weird sentence construction. Also, I don't think he quite "gets" some of what he is writing about; rather than render his subject matter opaque, it renders it more easy to comprehend, because he is trying to translate the concepts for himself. Start off with Tillich's The Courage to Be and Bonhoeffer's Letters and Papers From Prison; they shouldn't be too hard to find. Rubenstein's After Aushwitz is, I believe, out of print. Some of his other work, The Cunning of History and The Age of Triage is available through Beacon Press, the UU publishing house in Boston.
Before I get to the deeper theological concepts of immanence and transcendence and your own trouble with them (believe me, I have trouble with them, too), I want to address specifically the term "supernatural" and why I find it meaningless. I have said before that the word implies a clear understanding of "nature" in order to insist that something is "above" or "outside" the natural order. Yet, what constitutes "nature" is never clear. Earlier this week I posted a couple interesting recent scientific findings that deal directly with this issue. One of them is the black smokers, the vents on the sea floor where gas is vented through holes in the earth's crust. Because they occur so deep, the water does not vaporize, but exists in a superheated state, usually around 400 degrees Fahrenheit. The area around the smokers is not only superhot, but highly acidic due to the presence of various chemicals in the water. Had anyone said there was an entire ecosystem based upon the existence of these vents in the early 1970's, most biologists would have laughed. Simply put, the combination of acidic water and temperatures far in excess of the boiling point of water would melt the protein bonds that enable life. It is quie literally impossible, by a previous understanding of what is "natural" for these vents to support even bacteria, let alone complpex organisms.
Yet, they do. Through the arduous process of natural selection, organisms have developed that can live quite happily in an environment that is, at its most basic level, hostile to all life on earth. Indeed, the creatures that exist at these smoking vents could not survive elsewhere on the planet. While certainly now considered "natural", because they exist and thrive, they still defy any previous understanding of the word and its application.
Another quick example. In 1980, the Voyager I spacecraft did a flyby of Saturn. It took thousands of photos, making many remarkable discoveries. One of those discoveries was the existence of a pair of small moons orbiting just outside a thin outer ring. Called the "shepherd moons" because their gravitational interaction with the particles that make up the ring they border seems to keep it as it is, one of the things they do that astonished those who studied them was they orbit in helixical fashion. That is to say they spin around a common center of gravity perpendicular to the orbit of the planet. One of the mission scientists was quoted in a 1981 National Geographic magazine article as saying that this violated several laws of orbital mechanics. Yet, it was apparently the laws that were wrong. Again, this would have been considered "supernatural" before its discovery.
All this is not to argue the whole "God of the gaps" nonsense. Rather, it is to point out that before we can be clear about what is supernatural, we should be clear that we don't even know what constitutes "nature" in a way that is either clear or consistent.
As to your lack of comfort with my use of the term "participating" and the inference that, prior to this participation, God was somehow external to human, and presumably other, life. I suppose I should confess here that this was a clumsy way of trying to intimate one of the deepest mysteries of Christian religious belief, what has traditionally been considered the dichotomy, or to use a favored theological term "dialectic", of immanence and transcendence. These are fancy words that refer to the strange, contradictory idea that God is, of necessity, "outside" our understanding of time and space, yet we confess an experience and understanding of God as intimately involved in time and space, indeed in our very lives. Tied to these thoughts are the notions of grace, the incarnation. Lying behind these is the whole covenant tradition going back to the Hebrew people escaping Egypt (we do not need to deal with historicity of this event at this point; it is enough to speak of the tradition as it applies to their own self-understanding).
The kind of penentheism you offer is a common response to the dialectic. It is a difficult position to maintain, and can elide far too easily in to pantheism, which is a far less preferable answer to the conundrum. After all, the red maple tree out front, the small hive of bumble bees in the ground under our picture window, and my St. Bernard usually do not show any sign of divinity with which I am familiar. Yet, one can affirm a certain divine presence within them, yet separate from them. We do not need to get in to the whole "soul" phenomenon here; suffice it to say that this is both a problem and a mystery. We are here at the deepest, most basic roots of religious belief of any type. The liberal Reformed theologian Friederich Schleiermacher referred to it as the "whence" of the felling of absolute dependence human beings experience. Part of the genius of this formula is that is boils down so much of both Christian thought and life to a single word that is generic. He said we call this "whence" God, and spent close to a thousand pages unpacking this in a Christo-centric, highly Biblical fashion that nonetheless denied much of what went before in the tradition (if you're thinking of reading his Glaubenslehre, forget it; try his On Religion: Speeches to its Cultured Despisers, a series of sermons he published in 1799). Schleiermacher gave short shrift to anything we might call "supernatural", yet nonetheless affirmed the most basic Christian claims that, in Jesus, God was present in human life and history in such a way that the relationship between humanity (I suppose, to be all pomo, I should add "all of creation") and God was radically altered.
Part of the problem with delving in to this realm is that we are the very heart of the mystery. There is no solution that is adequate. One can even deny the mystery has any relevance or meaning at all. I am not suggesting one should accept it. I am saying that you should be aware that, right here on this point, we are standing over the abyss that, for Christians at any rate, offers only one lifeline out - profession of Jesus as the unique embodiment of the Divine love and grace for the world. I do not subscribe to any traditional metaphysical formulae for unpacking that particular phrase, yet I stand by it nonetheless because of that "whence" that Schleiermacher spoke of. I am quite comfortable ignoring the "two natures" theology, the ontological status of the incarnation, and those sorts of questions because, to me, they ignore the reality of the experience that is too deep for explanation, too profound for language to capture.
Finally, I would say that I do not want you to believe anything. I am offering these responses as a guidebook because, it seems, you are confessing (!!) that you are open to certain possibilities that you might not have been comfortable with before. Even if you decide to chase some of these ideas down, and decide they are as nonsensical after dealing with them as you did before you did so, that's OK. I am far more interested in the process than in any result. Also, remember this as well. Pretty much every answer possible - including outright rejection - has been given over the course of the history of the Christian Church. Your answer will hardly be new, but it will be your answer, and that is what counts.