As we have very little information on who Elidibus is possessing and who the mysterious figure in 3.1's stinger was, this is basically just throwing shit at the wall and seeing what sticks. Normally I refrain from this, but let's get cracking!
Mysterious Figure
-We know nothing about this figure, except that s/he is angry that Zenos was being discussed as "not dead" and was wearing white. The glove s/he was wearing was also visibly not that of an Ascian, so it can be safely assumed that the Mysterious Figure was not Elidibus. We also know that Zenos was buried after his death, though one of M'naago's comments during 4.1 suggests his corpse was exhumed and beat on by some of the less noble Ala Mhigans. That doesn't leave much to work with, but here are my hypotheses:
1. The Mysterious Figure is the real Zenos, and the one we fought was a clone. The Allagans had cloning technology, and Garlemald prides itself on reverse engineering Allagan technology. Perhaps they got ahold of the cloning tech and used it to clone Zenos... perhaps augmenting him in the process, which could explain the monstrous strength Zenos displays throughout Stormblood.
2. The Mysterious Figure is the real Zenos, and the one we fought was an impostor. Kind of. The idea behind this is that the real Zenos was not the warrior Varis wanted him to be, so he found some boy (an orphan with the temperament Varis wanted) and raised him as Zenos, similar to how Princess Garnet from IX wasn't the original but adopted the role after the real one died.
3. The Mysterious Figure is a sibling of Zenos. See: Larsa theory.
4. Unknown, or the Mysterious Figure has no blood relationship to Zenos.
... I still think there's too little information to draw a conclusion on who Elidibus has seemingly possessed. While it's easy to jump to the conclusion it's Regula based on the fact he is the only person Varis is stated to have cared about, that's... jumping to a conclusion. We know nothing about Elidibus' host beyond that their true identity shook Varis, and I don't think that's evidence enough to suggest anyone in particular. As always, it could be someone we don't know yet...
