"Before I Rise" was written with Aoi's death in her mind.
Aoi was Ruka's first true love interest.

>>527828309
It could be that while Bungo is Wakki's childhood friend, Bungo died a lot earlier while Wakki lived on for several more years.
How Bungo's body was found and narbified would be one hell of a mystery though. It would imply that she was either dug up or stored somewhere, then narbified alongside Wakki.
And as to why they don't think about that age gap, probably the same hoohaa that made them not think about what year it is.