So by then it’s obviously gonna be too little too late, given what we know from the spoiler.
In the more immediate present Eli, in addition to serving as the mc’s jailer is participating in tortuously painful “healing” procedures on the orders of Thalia all so the mc can use the parasites more, which is also a tortuously painful experience. So, yeah, real good vibrations in Arcadia.
Your return from Aulain had been followed by a long, tumultuous recovery period, in which Eli worked tirelessly with Tahlia to heal the damage that had been done to your mana ducts. Though the pair tried their best, it had remained a painful and exhausting experience, and you had frequently wondered if you were staring death in the face once more. The “healing” procedures Eli and Thalia use are so incredibly painful they really can be considered torture, especially since they are done only so the mc can use the parasites more often, an experience that is also torture and is then gonna require more extremely painful “healing”.
A lovely little cycle of pain and torture that really makes them little different from Thaubal except that Eli feigns bedside manners.
I guess that is the way the mc might finally go mad enough and snap to get Eli’s spoilered ending.
It doesn’t seem to be a want…really the spoiler seemed to indicate that by that point the mc is completely crazy and then Eli thinks it’s a good idea to finally come around and genuinely be on their side again, when the mc no longer cares for anything or anyone, including Eli. So, yeah, great choice Eli. Thanks for nothing 
I hope by that ending we find some way to torture the parasites themselves, cause it seems that may be the only “fun” that would be left for my mc by that ending. Particularly since it seems we’re never gonna get the parasites out. 
And the gleeful way they talk about knowing everything sister did to the mc without even shred of feigned empathy or sympathy. Yeah that really makes one feel welcome.
Code dive a bit and you''ll see that is not true in the strictest sense
Oh, it is even worse than that, as the Archangel route that is in progress has the mc forced to outright help one of Mireil’s former allies in Salantira…aka one of the people who helped her take everything from him.
So, yeah, Thalia has at least one non-Arcadian ally and its one of the vilest former henchmen of Mireil there is and they force the mc to help her. Someone he may finally want to kill for real and of course that is a person Arcadia is gonna force you to help. Fuck any sort of justice for the mc. 
in progress code spoiler:
She is a seasoned combatant, one who served the old Grand Duke faithfully as a mage of the Osirian. And that faith has landed her and her household in dire straits after the war, when the Grand Duke’s faction were revoked of their powers by the dozens—and the countess herself had only hung onto her title by the skin of her teeth.
She had been a client of the Archangel, before the latter deemed her to be of insignificant informational value. The Pavillion’s doors ought to have been closed to her for the past month or so—and yet there she is despite it all, standing before you when the Archangel’s dogs should have kept her out.
In the meantime any of Yu’s more reasonable suggestions about gaining better allies who are not vile former henchwomen of Mireil’s are not welcome because god-Empress Thalia refuses to consider anything except her own crazy plan.
Except that spoiler we’ve got says that in Eli’s ending it literally does, as it requires a high enough hidden madness stat to even be able to choose it, with probably no option to spare Saine and Ilya I guess cause the mc will be too mad for that, Yay! 
Full text of the tumblr spoiler post on the Eli route
Big spoilers under the cut. I’m talking… huge . Endgame huge.
In one of the planned endgame routes, you will be able to kill the entirety of the core cast, bar Eli (who will reluctantly assist you). It’s a rather short route (and will be written to feel like a ‘bad end’), with very specific triggers–one of them being that your mage needs to consistently pick options that add to a hidden ‘madness’ stat post-resurrection. (emphasis mine)
So, yeah the author himself says that to even get to that route the mc needs to be basically completely insane. But hey, at least that is the time Eli will come around to genuinely being on the mc’s side again. How nice of them. 
It is billed as a friggin’s bad end. While the mc deserves to be on their own side the only way that is actually gonna happen is for a short, unsatisfying and completely pointless bad end that requires the mc to go batshit insane. 
The other options are to be a slave in Arcadia or to become a servant for Leon again. So mc never seems to get any real agency of their own or a way to work towards any personal goals they may have, not even in the completely pointless bad end as that requires them to be insane.
Then the mc is screwed no matter what is his life will remain unpleasant and painful.
There’s only one person in the mc’s situation the mc, as both Eli and Sister only have/had one passenger they are/were actually able to sustain without too much discomfort. For the mc the parasites are quite literally continuous and ongoing torture and now they’ve got yet another useless parasite tagging along with that Earthworm thing.