I was also in shock when I realized that the first D-Mail saved Kurisu... but there is also another possibility, maybe Okabe doesn't have to choose, I mean it was never said that it's 100% certain that there are only two master worldlines. The Beta one is the original one where Kurisu dies, no time travel craziness, no SERN dystopia, alright. Then wee have the Alpha worldline which was supposedly created by the single initial D-Mail that actually triggered all the massive events.

There are just a ton of variables! It's very likely that Mayuri stays alive in Beta and Kurisu dies for sure, though Okabe also assumed there will be no SERN dystopia in Beta, how can he know that? There's no way to tell, after all John Titor still existed in Beta on the internet, this is proven by Okabe after the first few D-Mails where he asked Daru where his John Titor books are and since at this point Okabe wasn't fully aware of how things work yet it must mean John Titor existed in Beta too of course.

The next thing is the divergence meter. Wee could see relatively massive changes already, Akihabara changing, Ruka's gender too, on a worldwide scale these are really minor changes but if you think about it, all of these were only 0.xxxxx changes. What would happen if the number changes by 3 or 5? Maybe a Gamma worldline will be created where both, Mayuri and Kurisu are alive, Okabes ultimate goal isn't really to return to the "original" worldline, it's just to save everyone and everything from going down the drain so that isn't that impossible.

I put the original in quotation marks because the starting worldline of the anime isn't entirely unaltered too, in that flashback when Okabe was younger wee saw his Reading Steiner activating for the first time when he had that fever and assuming worldline shifts can really only be triggered by machines and not strong emotions there has to be a reason for that aswell, maybe that was the shift that moved the world to Beta in the first place, in turn killing Kurisu till the first D-Mail was sent witch switched everything to Alpha.

Plus I'm still thinking about why the first D-Mail which only had a garbled "Makise Kurisu stabbed" caused a master worldline shift from Beta to Alpha, I mean that message on Darus phone which he thought of as sPa/\/\ shouldn't have had such an impact saving someones life, there must be more to it. Of course one could argue it was the the first time travel activity, that's why but that doesn't work since nobody but Okabe with his Reading Steiner would notice the time travel activity thus it shouldn't have altered the world at all. And like said, with John Titor on the net and Okabes fever experience there was already time travel activity on the worldline which weakens the first D-Mail even more.

Of course with only four episodes left there isn't really much space for story plots left but I say lets lean back and see what happens, neither Mayuri nor Kurisu are certainly dead yet^^