The number of creationist doctors who recommend and give flu shots every year would tend to discredit your concerns. Again, creationists, even the hardcore ones, do not deny natural selection or adaptation. The thing the anti-evolutionists deny is that one species can evolve into a new, different species. None of the things you've mentioned are things creationists reject. At least not any creationists I know, or whose thoughts on such matters I am aware of. Granted, I don't spend much of my time seeking out creationist views, since I find them to be wholly without merit. But I do know several reasonably intelligent creationists, and none of them reject flu shots or hereditary disease, or the means of studying or combating either the flu or hereditary disease. If anything, they would just have some rationalization of how that doesn't disprove their creationist views. Rationalization is how they deal with any apparent contradictions between their profession and their religious beliefs. They rationalize that what they reject is not also what they assume to be true in their every day work. That may be wrong, but it renders their false beliefs innocuous as far as any potential danger to patients is concerned.
As for your more general point, I think you greatly underestimate both the power and the ubiquity of compartmentalization and cognitive dissonance. Those things are not in themselves indicative of abnormal psychology. If anything, their absence is abnormal, at least if normal is taken as a synonym of "common." In fact, their complete absence in a person is closer to "unheard of" than it is to "normal," and might even be detrimental to a person's well-being. I may develop an opinion on that if I ever meet someone who does not exhibit both to at least some degree.