This may seem random, but was it ever explained how you actually calculate when and how someone dies from the numbers that float above their head? I don't know if this has been asked before but I have never been able to figure out the mathematics behind knowing what the numbers under their name from the shinigami eyes mean.
According to an interview that Ohba did there is supposedly a way to convert the numbers into a number we understand, but he “forgot” what that conversion was. Also I think Ohba only ever applied that conversion to Light’s lifespan so everyone else’s lifespans that we see in the manga are just random numbers, iirc. I may be misremembering this part of it, that said. (Would greatly appreciate if someone who actually owns HTR13 could confirm or deny this).
As for the cause of death from when a person reaches the end of their lifespan – that isn’t set in stone and it seems to be purely driven by circumstance. For example, when A reaches the end of their lifespan, they commit suicide. When Soichiro reaches the end of his lifespan in DN, he appears to die from the injuries he sustained during the Mafia raid, but as it seems unlikely that he’d really die from that if he were never involved in the raid, he would have likely died from some other cause – ironically enough, probably a naturally occuring second heart attack. All that’s really certain is that unless a shinigami intervenes (as Gelus does with MIsa for example), the person will die from whatever cause makes most sense at that time.