[HurricaneHige]

I'm not sure if I'm correct, but I'll take a stab at this:

Basically, there is one world. Unlike the "parallel world" time travel that you were probably thinking about. The progression/history of each timeline is set, but it's not certain that the "world" will travel to it...think of it like an eroge, there are set "routes" a player can take, but a player may or may not take it. If you choose girl A you will not walk down the girl B path, or you won't get the harem end, etc. What Okabe can do is keep his memories from one timeline to another, kinda like the CG mode, or system file in an eroge where it keeps all your progression.

So, in ep1 Okabe made the world traveled down "route Alpha", thus resulting in a dystopia, and Mayuri dying (bad end). So undoing all the D-mail, sent him to "route Beta", WW3, and Krisu dying (also a bad end). Basically, by going to the Steins Gate timeline, it would mean Okabe is forcing the world into another "route". The existing route of Krisu and Mayuri dying does not disappear, it is in the "system", but also "untrue" as the route was not taking, therefore it is "not real" just like in an eroge, you cannot simultaneously achieve both a true end and a bad end in one play-through.

Oh lord...this is gonna be a long freaking week, wtb time machine.

EDIT: By the way, the above theory is in direct conflict of the theory the real world John Titor suggested, which seems to be what SG is based on...the only problem is that if the parallel world is indeed true, then Okabe's Reading steiners is contradictory. More on this on my blog, don't read it unless you have seen up till ep23.