Interesting theory. I thought it was pretty heavily implied that the Stranger was older than the Golden Age? I thought she had seen the pattern of the Dark chasing the Traveler over and over and was trying to break the pattern by helping the Guardians. It's been a while since I've looked through the lore, so I could very easily be wrong. I love some of the connections and deductions you make though, so I hope your theory pans out!