A primal halted the Empire's advance after Ala Mhigo?

[Enkidoh]

Whether or not it was Ramuh who stopped the Garlean advance or not, I DO remember a mention of Ramuh (not by name) being summoned before the events of 1.0. I wish I could remember if it was in one of the missions or if it was just one of the things a sylph floating in the Waking Sands said. It was something along the lines of 'once before we called the primal one and he tainted the hearts of the ones who called him. These ones will not do so again.' But unless someone can provide the text, my memory should be taken with a grain of salt.

Not exactly - during the first mission for the Path of the Twelve (Together We Stand), the sylphs indeed mention they will not summon their Primal, but they never mentioned that they had ever actually summoned it in the past - it was because they saw how summoning Primals affected the amalj'aa and the Ixal and realized summoning corrupted those who did so and didn't want that to happen to themselves. It's what caused the schism in sylvan society in ARR too - ironically they didn't even mention Ramuh by name at all (it was never confirmed Ramuh was even the sylph Primal until ARR). This is the dialogue from the above quest on the subject:

Flaxio: "These ones thought of fighting. These ones thought of many things, but the price... the price is too great."
Almxio: "These ones will not summon the Primal."
Zoxio: "The Primal reaps more pain than good. That One is best left undisturbed."
Path Companion: "The Primal... yes... Had the sylphs summoned their god, the Empire would already be preparing to enact their revenge on all of Eorzea. Thank the gods they didn't."
Diluxio: "Metal ones are the lesser evil. It is the Primals who darken hearts. Primals who poison minds! See the feathered ones. See the scaled ones. All are changed!"
Path Companion: "I have not noticed any great change in the behavior of the other beast tribes. They seem familiarly bestial, though a touch more highly strung, perhaps."
Diluxio: "Woken ones have eyes but cannot see! Woken ones refuse to see! Woken ones are blind!"
Path Companion: "We saw well enough to save the kin you failed to protect!... I'm sorry, I let passion rule my tongue. Though you doubt us, I can assure you that we saw the atrocities committed against you by the Empire all too clearly, and that we see you as friends. Friends against a common enemy."
Flaxio: "The metal ones are not the only threat. No. Not the greatest threat."

And of course, it should be noted that none of the beast tribes actually couldn't summon until the Paragons (ala, the Ascians) visited each tribe in turn and instructed them on how to do so - the sylphs mentioned the Paragons even visited them and told them how to summon, but that they didn't want to due to the reasons I mentioned above. When the Paragons appeared to each beast tribe is anyone's guess though, but I'm liable to assume it was probably some time around the time a 1.0 player's character first appeared in Eorzea at the start of version 1.0''s storyline (around 1572 6AE I'd imagine) - but no doubt one of the Loremasters will be able to confirm that.