Do you have any sources to back these claims? Also, what specifically do you mean by "half the casualties were instant"--is it that "of those who died, half of them died instantly" or "of those killed and injured, half of them received their injuries instantly". Or is it some other thing?

I think you're falling into exactly the sort of trap I was talking about, that the enormity of the devastation is so unimaginably great that it's difficult to imagine what it would actually be like, and to (somewhat lazily) conclude "well, it'd probably be instantaneous". But, for example, this analysis doesn't support that idea at all: https://thebulletin.org/2020/08/counting-the-dead-at-hiroshi...