Why didn't we like this?

It just means that you get to choose abilities from previously leveled jobs as opposed to just having the abilities of the current one available like in FF3.

why do airy and anne want to summon their demiurge?

and why was tiz's hometown the center of destruction?

They're their spawns.
It was just the spot where things go kaboom. Shit luck for Tiz.

For me it's how Edea is still hung up on Ringabel and has no interest in her world's Alternis.

you'll have no fucking idea of what's going on on the final dungeon.

No it's fine, the characters discuss what's going on in the dungeon each time they stop to have a fun little chitchat before the end of the world with almost no sense of urgency.

NO THEY DON'T THEY JUST TALK ABOUT FOOD