Have you read all that and wondering what's going on?
This is actually a flashback 30 years ago, to maybe a few weeks after the first spirit's arrival. Some time after this, Mana would be captured by DEM, and Shin would be gunned down trying to prevent Mio's capture.
Because they are same people. Shin died from the gunfire and Reine absorbed his body and made him new as a baby. So Shido is Shin's reincarnation. Mana didn't die so she is still same person. (Yes, she is an adult, but somehow maintains her youthful appearance). Reine gave birth to Shido and gave him to someone else and hoping to reunite with him as he becomes an adult. Reine wants Shido romantically btw and that's what set the Date a Live plot going.
Because they are same people. Shin died from the gunfire and Reine absorbed his body and made him new as a baby. So Shido is Shin's reincarnation. Mana didn't die so she is still same person. (Yes, she is an adult, but somehow maintains her youthful appearance). Reine gave birth to Shido and gave him to someone else and hoping to reunite with him as he becomes an adult. Reine wants Shido romantically btw and that's what set the Date a Live plot going.
You're a little off the mark there.
Mana didn't die or age because she was held hostage by DEM, hoping to exchange her for Reine, or Mio as she was known then. But the exchange never happened since Shin was gunned down and Mio escaped with his corpse, so DEM held onto Mana, keeping her in hibernation and surgically altering her to become a wizard, which shortened her remaining lifespan to just 10 years, and released her only when Shido was known to be actively sealing spirits after being adopted into the Itsuka family.