I don't think it's appropriate to read later documents back into earlier ones when it comes to establishing what people knew and when they knew it. You might be able to "get away with that" theologically, but not logically.
In other words, the fact that Titus says there was a promise of eternal life before this world/age began does not establish that those who lived through Genesis knew it. I think you concede that point, but I wanted to make it more explicit.
Regardless, we know Job believed in a life after this one, and we know he expected it for himself. We have no idea how he knew that or how he could "claim" it for himself, there being (probably) no "scripture" at all at the time he lived. But clearly it was an early belief.
If you believe all scripture to be inspired by God, then I agree that Titus gives you a Biblical answer, of sorts, to my questions. But if you don't (and I don't), then you have to go chronologically by what's written and when to determine when and how this belief in an afterlife (and, by extension, a belief in salvation and its "requirements") emerged.
P.S. The mods received a couple of complaints about a post on this thread. It was my belief that the post in question is gently poking fun at the discussion, not disrespecting it. Either way, it's not really off topic (and I'm usually pretty aggressive about what's on and off-topic, particularly when I'm engaged in the discussion).