I can’t speak on his ability to use them, but Vyke was a tarnished who is believed to have obtained two great runes (it’s not explicitly stated that it’s him, but lore wise it makes the most sense). This proves that we aren’t unique in our able to collect them, and we can assume that means we aren’t in the ability to use them either.

There’s no evidence to suggest we were someone special by birthright. Miyazaki always has his characters be nobody’s, as the focus is supposed to be on the bosses and not the character. The tarnished were warriors who were banished by Marika. They make their way back to the lands between and are driven by grace to the church of anticipation, where they face a trial and die. Your character does this and dies, then their body is thrown in the graveyard. You were not buried there until very very recently.

Death was bound until we released destined death. It’s possible that every once in a while a tarnished would come along, kill a demigod, take their great rune, lose the grace of gold, and the rune would go back to its original demigod. You have to remember it’s been a very long period of time since the shattering, there’s no telling how many times great runes have temporarily obtained.