Can't the MC dude just possess Elmia and try to fix her from the inside? He's not even permanently tied to Schneizel's body, only semi-permanently since he can't wander too far away from Schneizel. But otherwise he's perfectly his own individual, while Schneizel as well remains his own person.
That being said, the whole story so far has made the MC look like an especially stiff person who can't think outside the box and finds any differences in life compared to his expectations to be extremely difficult to cope with. So, even if he could do such a thing, he would never even attempt to try it. An unfortunate personality, which undoubtedly contributed to his death. It also makes him mostly useless for Schneizel, apart from the occasional suggestion.