The Aetherophysics of Soul Shenanigans (4.4 SPOILERS)

[Iscah]

• people who know Elidibus's 50-height Midlander is not at all compatible with Zenos's corpse unless his armor is a magitek exosuit with stilts in the damn shins...

Ohhh yes. That annoys me so much. I realise that changing his model would have given the game away, but it still just doesn't make sense for internal consistency.

Zenos Possesses a Resistance Fighter (4.3)
Status of Resistance fighter unknown. Did Zenos's soul get infused into a corpse? Did Zenos transform the corpse into himself and the game is being deceptive with character models as in the 3.0 stinger? (Notice the eye color.)

What was in the 3.0 stinger?

Anyway I think they'd simply avoid showing us his face if they were trying to be deceptive. (At least I hope they would.)

Instead it has what seems to be the very deliberate shot of Zenos's smirk on this unfamiliar face.

I used the character data to build an 'unmasked' version of him in the character creator (link). If it is his "true" appearance and not a placeholder, perhaps Zenos was able to transform the body somewhat towards his original appearance or perhaps it's just that at a character design level, they wanted him to still look somewhat like the original.

It's also possible that the face is correct but that the character is Garlean rather than Elezen.

Solus zos Galvus Returns (4.4)
Outward appearance of a young Garlean man. Varis kills this host body, and fleshless Ascian Solus appears, identical in appearance, nearby. Occam's Razor suggests Solus zos Galvus at some point in his life, transcended the flesh and became an Ascian soldier, but was not anywhere near was weak as the standard black-mask. Solus would then have transmigrated into a living body which he transfigured into himself.

I think an equally simple possibility is that the living human Solus was possessed by the Ascian (now also calling himself Solus) early in his life, but that didn't prevent him from physically aging.

But yes, overall I think it's too early to be making any definite guesses as to what exactly happened. Just hang on and enjoy the ride.