They can't be on Othard, as there's no reason for them to not be in Kugane as random NPC's and if they were on Islabard, some would need to be Garlean soldiers (as Islabard is a region completely dominated by the Garlean empire), which we should have seen by now (why would the Garleans keep soldiers who would be nimble in combat on the back lines?). This leaves the New World to the west of Eorzea, which most likely has not been invaded by the Garleans and has little to no lore on it (all we know is that the Mamool Ja came from there). Or heck, maybe even Meracydia... somehow.

It's not possible to make Viera playable because of the Ivalice raid. Everything in FFXIV follows a timeline and we'll be doing the Ivalice raid after we finish the Stormblood story. If SE care about the lore of their game, they cannot/will not make a race that doesn't exist (until after a certain point in the story) playable.