Kevin,
I am not an expert on the origins and development of multicellular life so I will leave that to others on this board. Responding to your second question is more up my alley.
You wrote: "evolution is a theory and according to the definition of the word from dictionary.com a theory is "a proposed explanation whose status is still conjecture and subject to experimentation, in contrast to well-established propositions that are regarded as reporting matters of actual fact." In other words, "an idea, notion, or hypothesis.""
I went to dictionary.com and note that you selected the second of five definitions of the word 'theory'. It's interesting that you chose the lighter, more colloquial definition. the one that is equivocated with "idea", and "notion". You avoided the very first definition that is much more appropriate here. I find that intellectually dishonest but will leave it at that.
I'll use the first definition because it's the definition used by scientists when they talk about a theory - "a coherent group of tested general propositions, commonly regarded as correct, that can be used as principles of explanation and prediction for a class of phenomena". Note the first phrase that mentions a group of tested propositions. That's the key to your whole set of questions. In order for a theory to be scientific, it must be testable and for this reason, science cannot incorporate the supernatural. The reason is simple. Science relies on the assumption that there are rules that govern the universe. The rules must be natural and regular. If you introduce a supernatural force, you allow for those rules to be broken at the whim of that force. That means that all tests of scientific hypotheses go out the window because if things don't make sense. You can just throw up your hands and say, "the God/Intelligent Designer/Shiva/Jujumonkey that I belive in wanted it that way" to any experiment. No, science MUST leave supernatural explanations out or it becomes meaningless.
You wrote" "Why is it a "sin" in the scientific world to allow the teaching of all options to the origin of life, such as intelligent design, evolution, etc.?"
Would you recommend teaching East Asian Geography in a Mathematics class? I hope you said "no". It's not appropriate. For the same reasons, we shouldn't teach non-scientific 'options' in a science class. I whole-heartedly recommend teaching Intelligent Design or creationism (from all religions) in a sociology class, but it shouldn't be taught as an alternative to currently understood scientific theories. See above for my explanation as to why these are not scientific.
You wrote: "Darwin didn't write the 'Origin of Life', he wrote 'The Origin of Species.' To the extent that variations evolve from different kinds of animals, I believe this because it can be scientifically witnessed. (For example; dogs, cats, birds, bears, horses, etc.)"
You're correct on the first point and I'm glad to hear it on the second. Darwin had almost nothing to say on the origin of life. The biological theory of evolution deals exclusively with change in organisms over time AFTER life has begun.
You wrote: "Yet, the idea that life was came about by some random order of things, that molecules and atoms fell into place is highly improbable."
How improbable and how do you know this? The problem with this statement is that we really how no idea how improbable the origin of life is. As an analogy, consider the probability of being dealt a hand of cards with four aces and a king. We can calculate exactly how likely that is because we can know everything there is to know about the deck of cards being used. We don't have that information when it comes to the origin of life. We don't know the exact conditions on the early Earth. Expanding to the rest of the universe, we're only just now beginning to calculate how many planets there are out there in the galaxy. Only then can we get an idea of how often life like ours might have developed elsewhere. Thus, we can say very little about the 'probability' of life developing. More about this can be found here: http://www.talkorigins.org/faqs/abioprob/abioprob.html
You wrote: "Couldn't the notion that there was a designer be concievable?"
Of course it is. But it isn't scientific for the reasons given above.
You wrote: "Darwin himself stated that to say the complexity of the human eye was by accident is non-sense."
This is probably the quote you're thinking of: "To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. (Darwin 1872)"
It's very common for creationists to quote this sentence and go on to convince people that even Darwin didn't think he was correct. If you read the rest of the paragraph from which that sentence was taken you'll find this: "Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. (Darwin 1872)" He then goes on to describe possible scenarios that could explain the development of eyes like our own. He even gives examples of organisms with the intermediates discussed.
Such selective quoting is intellectually dishonest and referred to as "quote mining". Numerous examples (including this one) can be found at http://www.talkorigins.org/faqs/quotes/
ject.html.
Good luck getting an answer to your first question. I know there are plausible scenarios out there. In fact, I know there are many examples of colonial organisms available. These are organisms that reproduce essentially on their own but work together as a group. I could easily imagine that specialization of cells to increase the efficiency of food gathering might lead in the direction of true multicellularity.