Personal theory, to work in the whole 'rewriting history' angle:

What if the calamities happen, everything goes to crap, and G'raha Tia wakes up in the future after his slumber, to a dead or dying world. In desperation, he enacts some kind of Allagan technology to trigger 'Time Compression' sending himself back in time to his former body, and picking up other Scions along the way via the echo to warn them. 'Let eons become instants' is him enacting this process, and while he does time is all jumbled up for him. Our 'called' scions are caught in this Time Compression with him, until he reaches his 'destination' and joins up with us in the main plot.

The history being rewritten, is only history to him, we never actually witness it, be we know it's what will happen if we don't act on it.