:;()$&@"\The Anime Pub/:;()$&@"

he actually let an "oh" slip, cracked me up.

I still think that episode 8 was a devastating moment for Okabe. What he experience was basically everything he fought for being reverted with no chance of turning it back (before Kurisu revealed that she had recreated the time machine). And when he HAD the option to go back to the alpha world line (not sure, I always confuse them), he was once again forced to make the choice between the two people he loves the most. Not to mention the fact that this should#ve brought back pretty much all the memories of alternate time lines that he tried to forget.

I wasn't quite aware of the amount of time he jumped back, or rather, what world line he jumped to. Gotta rewatch episode 8 then...
In the end, they're in similar positions, both have to sacrifice one of the things they long for the most (the future with Okabe in Kurisu's case and Kurisu in Okabe's case). I wouldn't want to have to make either decision.

He actually has to prevent himself from thinking that he cannot succed. Which means that he has to send a message to the past to prevent himself from getting to the conclusion that he can't change the future. So his only way to succeed is to make his past self go against everything he learned.

As for the thing regarding Okabe' Reading Steiner... the other world lines seem to get erased, everything in the zero AND the original hints at that. If it weren't the case, the the other people should't be able to retain any memories whatsoever. Rather than being erased, I think it's more accurate to say that they got erased AND overwritten, which explains the memory fragments. As with a hard drive, things that get eraded, but do not get overwritten can still be salvaged. That's how those faint memories happen, the events just didn't happen, but nothing happened instead, so they didn't get entirely erased from existence.

I hope that made at least some sense...

I'm of the opinion that there is only one world line. While it does switch "locations" (for a lack of a better word) from time to time as we have seen, it's the only one existing. As of now there's no way of proving the opposite, the only way to do that would be observing a different world line without interfering with the current one, which thus far hasn't happened. The only times Okabe observed different events in the world line, it was when he was inside it. As you said, everything else is nothing but a possibility, sometimes one that has been realised, sometimes it wasn't. Ones that have been tried are ones that can still faintly linger in the memories of other people if they had enough of an impact to burn themselves deep into the subconscious and this did not get completely erased. Which might explain why they almost exclusively appear in dreams.
That means, we can assume that Okabe is (again something you already said) an almost godlike observer entity and likely the only currently living being that has any way of interfering with different possibilities of the world line. Which means that the fate of humanity is decided by whether or not he becomes insane. And the only reason he hasn't thus far is Mayuri in my opinion. I don't know if she knows for certain or just has a hunch or something, but she appears to somehow know that the dreams she had about her own deaths were more than just dreams. Also, she's always somewhere close to Okabe in zero whenever he's together with a lot of other people (excluding university). The few times we see the two of them seperated AND something triggering one of his suppressed memories, he gets very close to a mental breakdown. At the very least, Mayuri knows that she is somehow connected to his suffering and she always tries her best to cheer him up. Without it, I think he would've lost his mind long ago.
Then again, I could be reading too much into things. After all, it's not like I know a whole lot about traumata and psychology in general. Still, I think it makes at least some sense.