I'm not 100% sure on this, but I think "Padjal" can be used as both singular and plural. Like moose. I believe "Padjali" is descriptive of something belonging to the Padjal. Like the Padjali language. Padjali magic. But you can run into one Padjal or two Padjal in the Shroud.
As for the WHM questline, it didn't go into a ton of detail, but the gist was that during the 5th Astral Era many White Mages abused their power (causing the Sixth Umbral Era) and it became a forbidden practice. And only select Padjal are permitted to use it and keep the knowledge of it alive.
-Raya-O-Senna, Seeds of InitiativeWhite magic and conjury are close relations, the two governed by the same laws. The former, however, places the utmost importance on healing and protection, and demands of practitioners the greatest intimacy with the Elementals. On account of this, its use is forbidden to all but those few who are deemed worthy. It is the sacred charge of we Padjal to ensure that the art survives, and in righteous hands.
-Oha-Sok, Lost In RageHast thou forgotten the pact of Gelmorra? The powers granted to the Padjal must not be used in conflict.
-Raya-O-Senna, The Wheel of DisasterThe Fifth Astral Era is said to have begun approximately three millennia ago. The ice age that ushered in the Fifth Umbral Era made the land a barren and merciless place, and man was pushed to the limits of his resourcefulness in the struggle to survive. Yet survive he did, through the discovery of magic as we know it - an event which marked the dawning of the Fifth Astral Era.
At first, man was well pleased just to keep the cold at bay and compete with the other races. But man is nothing if not an ambitious beast. It was not long before he sought mightier magicks, hoping to win greater glory. It was this desire that brought forth Black Magic, the arcane art of destruction, into the world. In order that this force of chaos be kept in check and balance preserved, at roughly the same point in history, white magic, the arcane art of succor, came into existence.
Emboldened by magic, man went on to reach the zenith of glory. But his hunger knew no bounds. Over time, even they who donned the white began perverting their powers for the sake of self-gain, and in this single minded pursuit scrupled not to sully the sanctity of the Twelveswood. In his pride and avarice, man brought down the wrath of the Elementals upon himself. A great deluge was sent to cleanse the land of his wicked presence, in the wake of which the forest rose to swallow up all that was not washed away. Thus did the Sixth Umbral Era begin.
So going over this quest text brings up a new theory. So let's say that the Elementals used to permit man to live within the Twelveswood during the 4th and 5th Astral Eras. Man discovers magic. Man starts to destroy and pervert even the purest magicks for personal gain. The Elementals wrath ushers in the 6th Umbral Era flood and the forests rise to bury all those within. The Elementals forbid man from using White Magic on account of this. Maybe this is why man is not permitted to live within the forest during the Sixth Astral Era. They are forced to live underground. Until the Padjal learn the art of Conjury to commune with the Elementals. This gives way to a pact that man can once more live amongst them so long as no harm comes to the forest and the Elementals gifts are not used for destruction or self-gain. Which would explain why Gridanians take nothing in abundance, kill only when directed by the elementals, and fear outsiders. Gridanians do not want the outsiders breaking the pact and causing another Umbral event because the Wrath of the Elementals does not distinguish between protector and defiler.
Going down this line of thought, neither Gridania nor Gelmorra would have existed in the 5th Umbral Era. Therefor the Twin Adders could not exist as we know it today. So the question becomes, who lived in the Twelveswood before the Gelmorrans?