What if the Narshe Expedition had succeeded?

[Greatermaximus]

If Terra was never freed from the slave crown, the story would presumably just be about the returners' constant struggle against the empire. Terra would never fly to zozo where they would never meet ramuh who would never tell them to go to the magitek facility where they would never have figured out magicite is where an esper's true power can obtained.

I cant remember how the party/empire found out about the cave of the espers though and whether tha insanity would occur without terra's presence in the party

Eventually the main party would have to fight terra in the second last/last bass fight

The Empire by Gesthal I assume found it in ancient texts. With his lust for power he invaded the Esper World. After that there's no way the information wouldn't be leaked by the time of the Narshe expedition.

I think FFVI has no plot hole. It might have one minor one somewhere but you never know where/when someone knows something. Especially in fiction.

If Terra had returned from the Narshe mission the Empire would simply keep using her imo.