They are brought back into a stage before their death, not necessarily in their prime although the Madara part was some kind of experiment on Kabuto's part.
The reason Edo Itachi is isn´t fully blind is because the Mangekyou's blindness cycle(?)
probably didn´t have time to take effect after the overuse of it during Sasuke vs Itachi, the blindness cycles increase in speed and blindness depending on how much and frequently the Mangekyou is used. As for the health I doubt it´s an issue since it was a disease and not a grave wound.
Sorry for long post xd