This ignores common law independent of statutes, and also isn't the least bit true.
Case law, with the exception of Federal District Court cases, is written by appellate judges who have a dispute in front of them. The dispute could be over a statute, common law rule, etc. All the case law does is interpret that statute or common law rule around the specific facts of the present dispute. For example, classic negligence law has 4 elements: duty, breach of that duty, causation, and harm (or damages). Some state common law (like Texas) have merged the last two elements. But in order for there to be relief in a lawsuit alleging negligence, these elements must be met. There's nothing statutory about the elements, though there are often statutes that add additional law to the common law. For example, there are both common law and statutory duties that relate to whether the allegations meet the first element of negligence. The common law duties don't supercede the statutory duties; they work in harmony.
if the DOJ believes that filing a tax return is "mandatory" why don't they cite the statute section that says directly so? What has been happening is that the DOJ uses U.S. v Sullivan, 274 U.S. 259, thus falsely forcing individuals to file returns under duress who do not meet the "requisite statutory definition" to file in the first place.
This has nothing to do with what you are speaking of. This, however, presents an issue for congress to remedy. The Supreme Court can only rule based on the written (and interpretations of) the statute in question. But it is up to Congress to remedy that interpretation through other statutory language that makes the ruling moot.
Your point here has nothing to do with lawyers using case law over statutory law. There is nothing magical about statutes; they only go as far as interpreted. I think if you went and set in on a law school class you would understand why you aren't making much sense. If your complaint is about judicial review, well, sorry, but you're 200 years too late.