Why did they think saving Isabella would save them? Doesn't make sense
Simply because Candyman tells them "Only new life can defeat death", they all become convinced that letting Isabella have her baby will save all of them (why???) and according to one of them, Death will be forced to give them all a clean slate. This makes no sense. They're literally just pulling it out of thin air.
I understand that they thought Isabella shouldn't have been alive to give birth, but that doesn't mean that death would give ALL of them a clean slate. At least in part 5, their logic makes a bit more sense of if they kill a person, you can take that person's remaining time, and they're taking your place, balancing things out, but even that's just guessing. However, it does make more sense than this one where they're just like, "Hmmm....hey, if the baby is born, we'll be okay! We're basing this off of absolutely nothing!!!"