One of the peculiarities of the law of Tzaras (~Leprosy~) is that someone completely covered in it is not impure. This is actually mentioned in the Torah.
The below is not written in the Torah. Rather it is in oral tradition, as is the story about what the brothers actually did.
A peculiarity of the law of capital punishment is if the entire sanhedrin votes guilty, the accused goes free [A separate mechanism deals with these people]. These are examples of facets of Jewish law that appear to be major loopholes and are baffling.
Similarly, a magically created golem animal does not require ritual slaughtering and you can have it with cheese. A magically created golem woman has none of the encumbrances a real woman has and you can even destroy her when you are finished. You could also have her with cheese. (You could eat a real woman with cheese- another loophole is that humans don't count as meat, assuming you killed her legally, i.e. ate her in battle)
It is obvious to anyone with half a brain that the brothers' use of these loopholes is distasteful and wicked BUT the "badness" of Joseph's report was that he told his father that the brothers were messing around with a woman, without saying that she was a magical golem woman.
The actions the brothers did, had they been done with real articles, would have warranted the death penalty. Joseph thus made capital accusations to his father.
As to artifice, God creates many things that resemble other things, like stick insects.