I can explain such expressions by stating that some things were meant to be left somewhat mysterious, and many things were explained in further detail later.
Which means the tutorial is context sensitive and Square is saying "People found this through research but this other thing is still mysterious".
Otherwise they would just shut up.

No dude. The wardens and milltary personnels of GALBADIA knew Squall is still alive and also nor will bother whether Squall live or not in Galbadia.
I mean,citizens outside of the prison.

Why didn't Laguna or Squall change their appearance or get possessed by "X" after they are announced in the GAME as alive and kicking.

Laguna don´t really dies and Squall recovered because Ellone send him to Laguna(when a person get hurt or healed on the dream world the damage or healing affects the sent person also).

Cause you said that everyone is lying and the whole story is a whole pack of lies.
My definition of lye on the game:

To say something about an event that contradict what the writers meant for that event.

For example:
Edea-Ultimecia is a sorceress who lives many generations ahead of our time.

But the writers probably meant for her to live on the past and they decided to create the event in which Edea tells that(lying).