Does a hero need to be alive to be saved into cache?

nt

Doubt it. You can, after all, finish the origin al Blizzard's comapigns at the moment your hero is dead.

Ofcourse you dont, a unit is a unit, wether it is dead or not. Storing unit data is not effected by its current life.

No, a unit's id keeps its value even if the unit is dead revived or whatever (except removed), what means no matter in which state your unir is.