There's all sizes and shapes of adherence and semi-adherence to kashrut. The most important thing you have to keep in mind is that it's not a question of reasoning in an ordinary sense: it's not about health, morals, or any of that, and it never was. It's a matter of law. Furthermore, the Law does not end with Torah: in traditional understanding, God gave the Law through Moses, but that Law requires interpretation and examination by human beings.
For example, the Torah (in the sense of the five books of Moses) does not say that you can’t mix milk and meat. It says that you cannot seethe (boil, simmer, etc.) a calf in its mother’s milk. But over time, the great sages (first priests, then rabbis) have determined that this narrow statement actually means that you can’t mix milk and meat. Because this is furthermore a matter of contact, as inferred from various purity laws, it appears that you have to keep your dishes and cutlery and stuff separate: the only cleaning that would be sufficient to allow you to put milk on something that has held meat would destroy almost any normal dishes. My point is that little of this is really debatable: the Law is the Law, and if you want to argue about fundamentals like milk and meat, you first have to master that Law in terms of knowledge, which means you have to read and master the entirety of the Talmud, which will take you (on average) rather more than a lifetime’s study. As to pork and shellfish, there’s no debate at all: the text is unequivocal.
Within the range of observance, however, there is room for some disagreement. There are disagreements about the strict interpretation of the requisite cleansing processes, for example. The strictest Orthodox interpretations entail that one cannot eat anything that has been prepared anywhere but a strictly observant kitchen. Some Conservative and many Reform Jews, however, consider that soap and hot water is sufficient, and as a result one can eat food made in any clean kitchen so long as eats only foods that are themselves licit. The question of whether meat must be kosher-slaughtered falls into this range as well. And then there is the matter of punishment: it is a serious question as to whether disobedience of various kinds of laws is a moral or a purity problem, and whether those two differ anyway. Thus some will argue that a minor and unintended transgression, as for example if you ate salmon (clean) off a plate that, unbeknownst to you, had once been used for shrimp (unclean), is no moral failing and thus no impurity; others argue that purity has nothing intrinsic to do with morality, and as such the situation in question is impurity and must be dealt with by appropriate cleansing. And on and on.
If you aren’t planning on obeying the Law, it becomes, at most, a question of what you consider “close enough.” If you ask me, though, don’t bother with cute rationalizations based on nonsense: you’ll often hear people saying that the ancient restriction on shellfish has to do with parasites, so modern food standards obviate the law here, and so on. BS, I’m afraid. There is not the slightest evidence in Torah to support this claim, and it is also strongly in disagreement with the range of what we find in Talmud. If you eat shellfish, you’re disobeying the Law, and you just have to decide whether that matters to you or not.