We do this for almost every other science, except evolution.

Do we? Could you show this for 3 other fields?

Either existence happened by chance, or by design. There seems to be no third or fourth way. We are limited to these two conclusions and nothing else.

Does the structure of a crystal come about by chance, or by design?
Does the demographic distribution of visibly identifiable subcultures within a diverse population come about by chance, or by design?
If I reroll stats for my D&D character until I get one I like, does the resulting set of numbers come about by chance, or by design?

Which is to say: I reject your assertion that the middle is excluded here.

With respect to evolution in particular, there have been many "less extreme hits" on the prevailing theory of evolution over the last hundred years or so, and they have resulted in changes to the prevailing theory of evolution. It happens all the time.

That said, if (hypothetically) someone is looking for is a "hit" large enough to necessitate a change to the theory on the order of positing a goal-seeking selection process, then yeah, that would involve a pretty extreme outlier. Which is to be expected, as it would result in a pretty extreme change.

It usually takes a large "hit" to cause a large change, in any field, be it science or theology.

Can you name any of these anomalies which "don't seem to make sense?"

There have been various evolutionary quandaries, where it's not clear how this or that organism evolved, but many of these have been resolved by further discoveries, which clarified the line of descent. There are some lineages that are still hazy, the evolution of bats for example, where our record of their lineage is poor because their bones are delicate and do not fossilize readily, but cases like these are not a source of confusion.

All the allegations I've heard of anomalies which supposedly do not make sense have been put forward by creationist or intelligent design proponents, and they were all based on some sort of misconception. There may be some outstanding sources of confusion in the field of evolutionary biology, but if you have any in mind, you'll have to clarify.

This is not intended to be a comprehensive review, but evidence against the theory of evolution could occur in such cases as

A: Minor: traits are found where it is unclear how they could have evolved by a natural process, but not highly implausible. These are found on occasion, but are generally resolved in time. Many traits where the evolutionary pathway was once obscure are now understood, so the evidence of any particular trait is weak.

B: Major: Organisms are found in the fossil record where it is highly implausible that they could have evolved from existing precursors. Fossil rabbits in the Precambrian fall into this category. Highly complex and unusual organisms with no apparent precursors, when we should expect their precursors to have left some sort of record, such as large organisms with radial symmetry and internal skeletons, would also constitute strong evidence.

C. Extremely major evidence. Events like seeing a monkey give birth to a human, or humans give birth to mutants with wings or the power to throw bolts of lightning. Any highly reliable observations of organisms developing major, complex adaptations in a single step. For all that many people with low levels of scientific literacy think that this is the sort of thing evolution entails, this is actually the kind of observation that would essentially falsify it outright.

As for whether "existence happened by chance or design," the meaning of this assertion is unclear. What do you mean by existence? Do you mean species as they currently exist? The phenomenon of life itself? The universe? The nature of the discussion will depend on what you actually mean, but I'll note that a dichotomy between "design" and "chance" is probably not an appropriate characterization. If you drop a rock, must the fact that it falls either be by design or chance? If something happens as a predictable, inevitable consequence of the rules regarding how things behave, it makes little sense to call it a consequence of chance.

Scientists tend to treat evolution as something that is essentially not in question because our evidence for it is overwhelmingly comprehensive. While there may be some instances of confusion, they are very minor in comparison with our existing body of evidence, so while further observations might alter our understanding of the process in some way, they do not threaten the entire edifice of evolutionary theory any more than the Pioneer Anomaly (which was in fact recently explained without any change to our understanding of physics) threatened to undermine the idea that gravity exists.

I would like to see the scientific community come up with more specific parameters as to what would be considered: A. minor damage to the theory, B. major hit on the theory, and C. evidence that would make the theory most likely untenable. We do this for almost every other science, except evolution.

I think we do this for evolution as much as any other part of science. In any, the judgment of the severity of a "hit" is possible if you understand the relevant concepts. An understanding of the concepts lets one see what separates minor issues from fossil rabbits in the Precambrian; what's a detail, and what's central to the theory - some things would necessitate a modification, and some would cast the entire theory into question. Think of what it took to overturn any other well-established theory in history, or what it would take to overturn relativistic physics.

More generally, if you have a whole bunch of evidence that points to one conclusion, it should take something fairly extreme to substantially sway you away from belief in that conclusion and make you re-evaluate all the accumulated evidence. (And there's a lot of evidence for evolution.)

There have been anomolies found in the fossil record that don't seem to make sense, but they are not deemed extreme enough by the scientific community to warrant any damage to evolution. The hypotheticals you have suggested are very extreme, do they have to be that extreme to warrant a hit on evolution or can less extreme finds also warrant questioning?

This seems to indicate a very confused thought process about how scientific theories work and are tested. A scientific theory that is wrong shouldn't have data that almost but doesn't quite fit. If evolution were wrong we would expect to see all sorts of bad data. Almost correct data with a few minor issues that we don't yet understand is exactly what one would expect for a scientific theory. To use a different example, there are some definite anomalies about gravity and how it functions (the apparent presence of dark matter is one of the more obvious examples). Will you make similar comments about gravity? If evolution and gravity are treated differently in this context, it is worthwhile to ask why. If the sole reason is that evolution has theologically uncomfortable implications, then what does that mean?