Karsten's post already disproved this - although my first thought on reading your theory (before seeing the image showing Aymeric was there) was that while I disagree with the suggestion that Zenos has possessed Estinien, it
could be possible that Zenos rescued us and was mistaken for Estinien.
Suppose we get rescued and delivered to the battlefield healers by "a mysterious Elezen warrior who just dropped her off and left without a word", and that description gets relayed to Aymeric, who has a pretty good idea who that someone would be... except how is he to know that we've got
two reclusive Elezen who care about our survival?
Disproved scenario aside, I had previously wondered (back in 4.3 when we first learned Zenos was 'alive') if such a thing might come up, because now we'd have two mysterious Elezen trailing us for completely different reasons.
Also, on the question of Zenos physically possessing Estinien, there's not actually much resemblance between them beyond that they're both using Elezen face #2.
For reference:
Estinien vs
Elezenos mockup based on his character data.
Also,
this chart I'd put together of a few different characters using that same basic face when 4.3 had just come out and everyone was freaking out. "Zenos posessed Estinien!" "Zenos possessed Haurchefant!!!" ...but no, they just look similar enough when you don't have them side by side to compare.
(Random observation I just noticed on my own chart though; Estinien is almost identical to Guydelot from the BRD quests. Only the eyes are different. Never would have thought of them as looking alike.)