Will Sin-eaters get the Voidsent treatment?

[Puksi]

And yes Vauthry is a sin eater but it is explicitly stated he is also a man, he is both, but like you said it does not make him any less of a sin eater but by the same token it does not make him less of mortal being. Sin eaters do not have a sense of self, they act purely on instinct; yet he clearly experiences emotions and all the imperfections of being a person.

Titania is also described as "corrupted" in this same way--a living being overwhelmed by the power of a Lightwarden. Still retained enough of their former self to be "sick of this cramped old castle", and still retained their form, but apparently now too murderous for even the Fuath.

Maybe it's more a parasitic relationship than symbiotic? The emergence of Innocence never looked like an actual transformation to me, as the face in Vauthry's chest sinks in, and when his back splits open, you can actually see Innocence's head/hair/back/wings inside. It gave the impression of some Junji Ito nightmare meat cocoon. The aether-like sparkles while it was happening suggested the sin eater just absorbed what was left of the host's body, but still retained that twisted version of Vauthry's self as the Il Mheg Warden retained Titania's.

I really wish they had explained things better than that footnote of a cutscene with Emet-Selch and the Former Mayor.

meol which is sin eater flesh

This part has always confused me. Sin Eaters throughout the game were shown to have no blood, bones, or meat. They dissolve into sparkles--the dialogues really stressed the sparkles thing iirc, as those would turn anyone caught in them into an Eater also. Even Tesleen, who recently turned, just poofed when defeated in Holminster Switch.

Did they just dust those buns with sparklies like it was flour? Condense the sin eater down like a soup?

There weren't enough people in Kholusia to "disappear" for twenty years to do all that, without starting a panic. And I'm still not sure why the extra step of bread was required. Meol was an optional dish for the rest of the Eulmorans, and if it was part of some plan, it was a terrible one. It just made some of them sleepy, and the others lurch around and mumble, lol.

Getting back on topic--Voidsent seem a lot more focused and less prone to random outbursts to the Light-corrupted on The First? So far, anyway. Zero doesn't seem a perfect parallel to Vauthry, as Zero isn't hosting another creature in her hitbox, unless maybe Zeromus is hiding under that cool glam. (But that would be too obvious!)