my hesitation in declaring him as Rasler was always the fact that the original Rasler in the lore would be in his fifties by now.

Drest's current age is what will help unravel this mystery-- exactly how old is he?
He should be roughly as old as Rasler, considering he appears to have fought the Garleans during the invasion of Dalmasca. IE, he was at least of fighting age.

Is she hiding with other members surviving members of family (and her resistance army) within the ruins of Lesalia?
This is an aside, but while Fran is operating the resistance from the ruins of Rabanastre, I was under the impression that this supposed Ashe was in Golmore with the Viera. In XII, after all, "Lente's Tears" was an artifact that granted passage to the Viera's home.