>>718361364
Chain of causality.
Mayuri has to die so Okabe hates SERN enough to create a resistance group, and that resistance has to exist so Suzuha has the means to time travel. That chain of events is what the VN calls an "Attractor Field" and the Universe is very autistic in making sure it happens as to avoid a time paradox. You can change things as much as you want and as long as the new timeline (or worldline, as they call it) doesn't break that chain, the Universe doesn't care, which is why something as drastic as Akihabara not being the Mecca of otaku culture anymore is okay but this girl in a blue dress has to die no matter what.
There are "oversights", however. if you make a change that happens to break the chain of causality, the Universe is unable to keep following it and "Fate" stops being a thing, at least until some retard messes with time travel again and creates a new Attractor Field. That's basically what Okabe does at the end, finding a loophole that breaks the Beta Attractor Field (where Kurisu dies and WW3 happens) and creates a worldline without a checklist the Universe has to follow.
>Why does deleting D mails delay her death for only 1 day?
IIRC (and I could be very wrong here) SERN is keeping tabs on global communications and can more or less guess what Okabe changes by looking at the D-Mails. At a certain point, they decide he fucked with time too much and raid the lab, killing Mayuri in the process, so undoing the D-Mails delays the "breaking point" by a day.
Honestly, it's just a meta thing to give the player a sense of progression.
