It is possible to have an irreducibly complex system come through natural selection. Cooption is an important part in Biology, and it deals with how existing features become adapted for new functions, which can form "irreducibly" complex systems.
Natural selection is a phenomenon embraced by creationists. Creatures can indeed learn to use existing features for new functions. However, macroevolution cannot explain how the features arose to begin with. Mutation and natural selection involve the
loss or shuffling of genetic information. As I said before, a man could never become a millionaire by losing a penny a day for 30 years. In the same way, a microbe could never become a man by losing one gene a century for 3 billion years.
Atomic Theory is as indirectly observable as macroevolution.
One can split atoms in the lab and observe results thereof, or see the effects of quantum mechanics. These tests can also be repeated. No one has ever observed life from a non-living chemical concoction or one instance of a spontaneous appearance of a new gene. These would be an indirect way of observing macroevolution. The microevolution we observe actually serves to
disprove macroevolution, for the reason I gave above. The genetic code is
losing information, which is the
opposite of what is needed to produce a man from a microbe.
John Paul II even admitted in light of more recent discoveries the Church needs to consider Evolution more than a hypothesis.
This is an example of how this pervasive philosophy affects people's religious beliefs, when the Pope does not even claim to have faith in the Bible.
Science does conflict with a literal reading of Genesis, but I can assure you that in no public school does a course mention how science is related to Genesis. Like you've said, it doesn't occur in a schoolroom, so the schoolroom doesn't decide which way people think.
Now, now... I give people a lot more credit for intelligence than that!

Even if Genesis is not mentioned in the schoolroom, people are perfectly capable of seeing the conflict between a literal Genesis and the theory of evolution. This has led some to abandon their faith, others to compromise it. Who has more knowledge and authority? God or the men He created? In many, many cases, people believe that men do, since they adjust the Bible to fit around evolution rather than adjusting their theories of origin to fit the Bible. However, as I stated before, in this social climate, I would not expect this to be brought up in a science classroom.
I agree about the models, but only as long as we stick to scientific models, which would exclude Creationism and ID. We can talk about what we know of Egyptian construction methods and say that people aren't really sure, but we can't say that Aliens or God put the pyramids there.
Why do you say that Creationism and ID aren't "scientific"? Do they not observe data and test against the theory? Just because the theory involves a supernatural Being? Evidence of this Being is just as observable as evidence of a "Big Bang," if not more so. Once again, what you are seeing is the clash between two belief systems, atheism v/s belief in God. Why should one have exclusive coverage in the classroom?
Assumptions are axiomatic, and the "many more assumptions" are proven (through microevolution) pathways that proponents of ID also believe in. The only difference in belief is whether or not these pathways combined to form macroevolution. If there was something beyond these pathways, like the Creator in ID, that would need to be another assumption added, violating Occam's Razor.
No assumptions added. Both models assume a beginning. The universe had a beginning at some point. The
axiom is "whatever has a beginning has a cause."
This is self-evident. What model fits best? That an eternal Being (who had no beginning, and therefore needs no cause) created the universe, or that the universe created itself, making the
assumption that it existed (in order to create itself)
before it existed. This is a logical absurdity.
Saying "microevolution leads to macroevolution" is not axiomatic. It, too, is a logical absurdity. It is, in essence, saying that the process of losing genes, if given enough time, will result in creatures with more genetic information.