how is that not proven correct? lmao, it is literally state in the shatrering books how the rules work, there is no confusion. It is even reinforced latter in one of the comics where Fenris face off his father.
The rules wildly differr exactly because what i said, it is changed from the original ones on the spot to adapt the fighters and the moment they are, because obviously, most of the time, they do not have enough time to proper do the ritual.The rules have wildly differed in many cases where they remained unstated and the only times that certain ones were adhered to specifically were ones where they were laid out before the event(thrall vs garrosh I think specifically said no magic). So, no, it is in fact how it works.
You need to have established rules in the first place so they can differ, change or adapt, you cannot change rules of something without rules.