What exactely happens in the Alliance Raids of Stormblood?

[Monsalvato]

Anybody can point me to a place where I can read a tl;dr so I can make some sense out of it?

I got there's this family of nobles from the Garlean Empire, who are good people, and own a flying airship and a theatre company where they play acts that are not actually made up from zero but based on the tales of their Ancestors.
The company owner's son and daughter look exactely like Ramza and his Sister, they are their descendants and get "possessed" by the original Ramza/Sister souls during the raids at some point.

Then there's Fran and other character from FFXII, which is... possible I guess since FFXII If I recall takes place many years after FFTactics (and, just like all Matsuno games, they all take place on the same world: Ivalice)

So I'm confused, does this mean Ivalice is not a planet on its own but it's actually a realm/zone on Eorzea? But we can't freely explore it, just through raids.



I dunno, I'm having a hard time trying to believe FFXII/Tactics took place on Eorzea/Hydaelyn/TheSource.
Thanks for your time regardless!