Here we call the science not law, but "derecho" wich comes from "directum" (rect, oriented to or by, from the latin) the "derecho" (that would be translated to english like "right") has four sources: jurisprudence, law (ley here, "lex" from the roman language), doctrine and custom. Between the four they cover every hole mentioned avobe by you, so there's no need to establish them in the law, because there's others sources that the judge, jurists and lawyers take in account, and that anyone without a minimal knowledge of the science will never know, therefore cannot make a true judgement in this complicated matter, that's not a question of morality or simple custom, but right, directum.
In England and Wales the law has three sources, statutes passed by parliament, common law, which, I suppose is the same as "custom", and case law where judges interpretaion of statute and common law binds subsequent judges to behave in the same way. This is complicated: society and technology change, so new situations arise, language changes, new statutes overlap with old ones, new judgements are made; in order to understand all this, you need years of training. "Anyone with a minimal knowledge of the science will never know it...", I agree entirely. However in most cases the issues are those of fact, and not of law. In England and Wales juries never make decisions about law. They only make decisions about fact. If the proposed system in Argentina is that juries make legal decisions, then you are right to be against it.

They are not making decisions entirely on their own. The judge is there to guide them. He or she will say things like "If you believe such and such a thing you should find the defendent not guilty. If, on the other hand you believe such and such, you should find the defendant guilty." If the judge does not give correct direction, then this can be challenged on appeal. I suppose an argument could be made for alternatives to jury trial for cases where facts are very complicated and involved, perhaps fraud cases. However in nearly all cases, both the law and the factual issues are straightforward.