So, of someone goes back in time so they don't die, they don't die right?

That's if their lucky...

When Nagisa died while giving birth to Ushio, Tomoya thought it would have been better if they had never met, that why she would still be alive. and so, it was kinda like they never met. and Ushoi lived at her grandma and grandpa's house. after awhile, Tomoya was regreting think that. but it was too late, so he thought. But u know how the light orbs give u a wish, if Tomoya (AKA the little robot doll in the ended world) colected enough light orbs, Nagisa could come back. and Ushio dies to, because if Tomoya and Nagisa never met, Ushio would not be there. but since he colected enoigh light orbs, they both came back to life. hope this helps. is a short, yes, Nagisa, and Ushipo both die, and both some back.