You are totally correct on all points - the gremlins in the First are not actually native to the First at all but only arrived with the Crystal Tower (a Lakeland sidequest revealed that). It makes you wonder what other creatures (both living and aetheric) may have also caught a lift from the Eighth Umbral Era Source to the First with it....
Also, the First does have other Voidsent as well - the tank role quests dealt with that (There are still some remaining voidsent from this incident found in places around Il Mheg, but the Flood of Light and the fae mostly drove them into whatever shadows they could find to the point they were virtually forgotten, the Sineaters naturally becoming a much more greater problem.specifically, prior to the Flood of Light, the former Kingdom of Voeburt's court mage was dabbling in voidsent summoning as a way of destablizing the kingdom so he could seize the throne, he was the one responsible for spreading the 'sickness' around the kingdom, turning innocents into voidsent demons while at the same time pretending to be helping Branden and the royal family find the cause of the problem.
The court mage's duplicity eventually is revealed, but at the cost of Voeburt's Princess Saludia being turned into a Voidsent demon herself, requiring Branden along with Ardbert and the rest of his party to put her out of her misery (after dealing with that damn court mage of course).
And yes, what is left of the Thirteenth is the World of Darkness, aka the Void.
That's technically what the original version of THM in the original version of FFXIV was like (version 1.0), it was a spellcaster that involved using Astral and Umbral aligned magic to heal by draining health and mana from the enemy and inflicting negative status effects - alas it was completely redesigned to be a better fit for BLM when the battle system was restructured and Jobs were added in the 1.20 patch.
Thus THM lost the drain healing (CNJ becoming the dedicated healer) and the negative status effects were mostly eventually given to ACN in ARR and THM became a fire, ice and lightning spellcaster instead (those spells originally being CNJ spells), the remaining legacy of the 'old THM' being the Astral/Umbral alignment shift mechanic for it's spells. And SE retained this into ARR which is the version of THM we have today.
Hence I do not see SE ever going back to that idea, it's in FFXIV's dark past that SE are very eager to move away from I'm sorry to say.
