For the benefit of those of us who aren't immersed in fantasy, could you share an example of a non-complex use of prophecy that doesn't serve as agency for either the protagonist or antagonist?
Well, there is the prophecy about the Witch King of Angmar in The Lord of the Rings. Glorfindel had prophesied that he would not "fall by the hand of man" (or as the Witch King says "no living man may hinder me" although he had already been hindered at Feathertop when his purpose was to kill Frodo and he didn't). Everyone took this to mean that no one could kill the Witch King, so nobody even tried. But the prophesy was fulfilled by Eowyn and Merry, two secondary characters -- as was the nazgûl himself for that matter. And it certainly wasn't something Eowyn thought about doing until she found herself in the situation of trying to save her dying uncle, and in the heat of battle. It's possible that she didn't even know there was a prophecy.
Given the era the story comes from, I think an orphan knight-in-training is probably about as egalitarian you're going to get. Anything lower than that and Camelot would be a work of social revolutionary fiction. A serf attaining any advance in their station would be simply unbelievable to anyone of the time.
None of which changes the fact that Arthur was not an "everyman" character as you suggested. As squire to Sir Kay and training to become a knight -- a privilege which the common man didn't have -- he was part of the ruling elite, although at the very lowest level. Did the common man or woman identify with him? That is very doubtful, especially since they knew the beginning of the story and were quite aware of who he really was. It probably wasn't until hundreds of years later that people began thinking of the young squire as a lowly character and were able to identify with him.
(Serfs could advance to freemen under certain circumstances, and to them it must have seemed a tremendous advance in their status. So for a serf to attain some advance in their station was both possible and believable.)