The thing is, Final Fantasy XIV has always taken an interesting approach to belief, in that it considers peoples' religions and practices completely valid despite them being proven false in the course of the story. Despite the Scions' knowledge that the Twelve are likely not deities they still chose to pray to them. Despite knowing that the creation myths behind tribal religions like the Ixal "false" and the god they worship "fake", the game does not go out of its way to paint their belief in that as foolish. People in Ishgard still believe in Halone, despite the church being brought down a peg.

In a lot of ways the game is of a "live and let live" mentality as it relates to religions and the origins of myths and such, so having the answer to these is not going to change much.

Now for the knowledge of talking to people after death, they do mention that, and very specifically say that it is something Sharlayan is keeping under wraps to avoid people going out of their way to do so.

As it stands, a regular Eorzean probably knows that the End of Days came and went (blasphemies were popping up all around, even in places that didn't have burning skies), and that somehow it was stopped, perhaps they even know someone that helped brave a shard of Dalamud to obtain materials to do something to stop it in service of aiding the Scions or the Warrior of Light in particular. I doubt they know specifics beyond that.