ugh I finished RE8 I'm late and going for a second round just coz I didn't find collectibles enough
HOWEVER I must make a theory (in case this was already theorized somewhere by someone I didn't know also link me I just wanna talk... 🤓)
This is my long TLDR theory of the post-Mold, or how the nature of the Mold is like in their hosts in relation to trauma and recovery. Please strap yourselves in and apologies it's on mobile....
Anyway there's already been hints that Ethan (and Mia AND Zoe) are going to have traces of the E-series mold no matter how they got 'cleaned'.
The best clue we ever have about the nature of the Mold and being clean from it is from
Lucas Baker.
This guy had monitored and 'taken cared' of Eveline for the last 3 years she (and Mia) are in the Baker residence, as well as being heavily monitored by that provided underground lab in the abandoned mine. Lucas had been in contact with the Connections for that long!
On top of that, he had been provided a 'vaccine' of sorts so he wouldn't be under Eveline's control anymore. However, nothing from the effects of the 'vaccine' (as far as I can tell and as far as was provided) makes it clear Lucas went back to normal. He's simply immune from Eveline.
However, it's undeniable he retains the nature of this E-series Mold. He sustained multiple, MULTIPLE injuries in RE7, including implications of being harmed by his "father" under Eveline's control before Ethan showed up/events of RE7. The infamous arm chop in the Dinner Scene reveals Lucas saying "... Not again!" which highly concludes Lucas is harmed like that by his own family.
When Ethan faces Lucas, Lucas is gleeful and self-mutilates himself in front of a camera, WITH HIS ARMS intact again, and pulling his own nails to reveal regenerative abilities to Ethan. It's also in this scene that Lucas has been pulling strings behind the Baker residence for 3 years, even with Eveline around. He's in charge and just pretends while Eveline plays dollhouse.
However, and it feels like a stretch, but there seems to be a time limit on how long the post-Mold (that's just how I'll call Ethan, Lucas, etc.) keeps their body intact and "human". I also think, depending on how unhealthy the body is, the strength of the Mold inside will degenerate over time, or in case of physical trauma, mutate.
In both RE7 and RE8, there's both a 3 year gap. Mia is infected with the E-series Mold in 2014 and 3 years later is finally 'cleaned' of it in 2017. The same can be said of Zoe Baker, who only received treatment after the events (End of Zoe).
Lucas got his serum/vaccine in January 2015 from the Connections. That puts him earlier, although not as early as Ethan and, to my theory, Uncle Joe Baker, who both got treated if they have the mutamycete almost immediately.
These two guys got immersed in the Baker residence for one night only, but the infectioness of the Mold is high (hence the area being cordoned by Blue Umbrella/BSAA) and Ethan was technically already infected by just going in the Baker residence, on top of being in contact with Mia again, having been 'chosen' by Eveline to be part of her family and up to the final part where he hallucinates a mutant Mia back in the guest house.
Despite being treated, I think the treatment degraded the Mold inside, but never fully went away. They're 'vaccinated', which is the best term I can come up with, and their bodies will react negatively depending on how much they got harmed.
Ethan and Lucas are both harmed a lot. Lucas is tormented by his brainwashed family and self-mutilates. In his final moments, he was finally fatally shot by Chris and Lucas transforms, triggering his Mold mutation. This part is heavily reminiscent to Lady Dimitrescu's Cadou mutation, as she is fatally harmed by one specific item (Death Flower Dagger) and triggered the Mold Cadou. (With Dimitrescu, we may attribute the dagger being closely related to Dimitrescu's genetic illness, but that's another theory for another time...)
Mia, Joe and Zoe are definitely also harmed, although the gravity of how they were physically harmed and/or how fit/healthy they are is up there.
I can only assume that Mia, having been the first Eveline has infected with the E-series and has been in captive for 3 years, has a LOT on her plate. She's also knowledgeable enough, just like Lucas, regarding the E-series Mold, as she was the Connections' operative. From the next 3 years moving to RE8, we were given proof of Mia's medications in the new house. So she's been recovering from the abuse and infection, and she doubled down when she had been pregnant! (Which is a miracle! Also, Mia has also a triggered mutation, that horrifying intro to RE7!! Who knows how she would've ended up if she let her mutation got worse or Eveline ordered her to mutate further!!!)
With Zoe, it's a little sketchy. We can assume she had been harmed by her family but she has kept herself isolated from them most of the time. In her notes, she does describe how she feels and the continuing degradation of her parents' minds and likely their bodies. From here, it's not physical trauma that's harming her but their degrading health. With Zoe being the only one who likely eats as normally as she can (if she has reserves of normal food [and from the clues in the house there seems to be a stash of some though supplies are low]), she's definitely running out of time. She had also calcified for a short moment, according to the Canon ending and in End Of Zoe DLC. With that, I can assume that the Mold was supposed to kill her, as ordered by Eveline but she pulled through because of whatever treatment BSAA/Blue Umbrella has.
Joe is definitely infected for a short time as well, and him punching/being in close contact with these Molded and the final Jack Baker Mold doesn't help his case being not infected. He definitely also sustained injuries trying to save Zoe.
With Lucas and Mia, they succumbed to their mutation (Mia, as ordered by Eveline, and Lucas through a fatal shot). Joe and Zoe never did, likely because Joe was infected in a short time and Zoe kept herself isolated as much as she could from Eveline.
Ethan is definitely a special case. He, Mia, Zoe and Joe are post-Mold, but Ethan is re-exposed to Miranda, disguised as Mia, and enters the Village likely infested with little amounts of the Mold. The watermill is under Moreau's dominance, which means the water flowing across has been tampered for so long. The Cadou pollen has likely travelled during spring season, and from the notes of that guitar maker, there were some people coming in and out of Beneviento's estate (before disappearing). And the main Mold itself, the Megamycete! That's been growing underneath the Village for God knows how long....
Ethan has been re-exposed and physically traumatized MULTIPLE times! Despite a stable body that got injured/mutilated so many times, Ethan was degrading during those intense moments of injuries/mutilations in RE8.
And never, never once did he mutate at all!! No triggering, nothing!! He, Lucas and Heisenberg have some similarities, in that they have a stable physique. Ethan's stability has been passed on to Rosemary, coupled with post-Mold Mia being her mother, which made her even more special!
(However, I will say, if Ethan's heart hadn't been pulled out of his body, he may have had a mutation. But Miranda took no chance and went for a vital organ. Come to think of it, a Mold hosting inside a living or corpse need to be intact as much as possible. This is further supported by the RE8 notes from Dimitrescu, Moreau, Heisenberg and Miranda... 😲)