I hope you'll forgive me when I tell you that it's complicated. I had a brief look just now and there are a number of important aspects and there is some confusion in your post. First of all we must be clear that the Jewish religion is based upon the Written and the Oral Law. If either of these can be said to have supremacy it is the Oral Law which we believe was given to Moses on Sinai along with the Written. So the interpretations are not arbitrary they are revealed. Quite how this ties in with the arguments over interpretation is another and equally (if not more) complicated question.
The point you cite from wikipedia is about the reason for a command. In Judaism we keep the commandments because God told us to. That is first and foremost. However, the commandments can be categorised into two groups - one for commandments for which we can provide some reason and one for which we cannot. But we always remember that whatever reason we give is not the true reason for that is known to God alone, but merely a "taste" something for us to think about while performing the commandment. (I use "taste" because the Hebrew word for "reason" is the same as for "taste" and this connection has been made by Rabbis before).
As for the specific question you ask about the derivation. I would have to look in the Talmud to be sure, but from what I saw briefly it appears to be based on a number of principles. One of the most basic is that what is in the Written Law is, wherever possible, not taken to be superfluous or for emphasis. Thus if the command is repeated it must be really separate commands. An explanation for the odd wording (referring to eating as cooking) is given that the command for eating is here different to other places. In other places eating is only forbidden if benefit is derived whereas here any eating is forbidden.
I won't pretend to be any sort of expert on the derivation of these laws but they appear to be complicated. What does seem true, though, (in this case more than others I've seen) is that the conclusion was known before the discussion. In other words, the Oral Law about meat and milk was known before the Rabbis of the Talmud discussed it (obviously) and the discussion of derivation is a post-facto one.
Apologies if this isn't clear but Judaism isn't clear - it require years of study to get a handle on so many basic concepts and decades more to become any kind of expert.
EDIT: About the shareholders - it is an interesting question. There are two points: 1) The nature of the benefit drawn is not clear. In this case I don't think that all benefit is forbidden so one would need to consult a competent Rabbi about the nature of the command. 2) Being a shareholder is an interesting question. We might ask how people can be shareholders of a company that operates on the Sabbath for instance. Again, this is a complicated question and not one I can really answer. I suppose it comes down to how big a share one has and how much influence that gives etc. There are lots of factors here, as everywhere.
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