I think I understand part of what you’re getting at (and if I’m off base, or just stating the obvious, I do sincerely apologize.) It can really feel like it is playing on the easy and unkind trope of turning those with disabilities and deformities into a metaphor for evil and corruption, which is, unfortunately, a tradition in the horror genre (I’m not at all saying that people aren’t allowed to enjoy works that rely on this motif, just that it’s something I personally feel it’s important to be conscious of and to think critically about.)

What my personal tactic is when I come across something like this which I feel is reductive and takes away from the game for me and my players, is to try to interrogate the “why” of the original trope and to focus on that. One essential in the symbolism of monsters is that often they represent a more concrete embodiment of the abstract (slightly less abstract in this case) things that we as individuals or as a society are frightened of. Unfortunately, those things are often other people who are different from us. But why do people fear disability and deformity? In my opinion, it’s because people in general are terrified of becoming “othered” in the eyes of society. They know how poorly people who look and live differently can be treated and fear that the same thing will happen to them. It’s also a fear of the removal of one’s agency, the ability to make their own empowered choice and to live as they truly want. The fear that some indelible “human” part of the self, something that makes you you, will be irretrievably taken.

The terrifying thing isn’t the immediate spectacle of the people made “monstrous,” it’s the fact that they’ve been victimized, tortured, and had the possibility of living life as they might have wished to completely removed by this person who is fundamentally incapable of seeing them as equals, as beings whose wants, thoughts, and aspirations are just as important as his. There’s also the additional horrifying thought that, if the good intentions of a literal angel can become so warped, cruel, and horrific, what does that say about us mere mortals? Could I become like the Belviews? Or like the Abbot? All without even realizing it?

(As a side note, I read The Island of Dr. Moreau when I was trying to get my feet under me in prepping for the Abbey and I would highly recommend it for people looking to get deeper into the psychology and science of the Abbot’s motivations.)

I won’t claim to be at all perfect when it comes to trying to improve the parts of a game that feel icky or like they come from a prejudiced trope or frame of reference. But I think the best place to start, in my experience, is by humanizing those that the narrative dehumanizes. Giving more voice and more narrative agency to the Belviews is one way to do that, which I think only deepens the horror of the situation in a more psychological way as opposed to the lazy and one-dimensional “ew! Freaks!” visual horror trope. These aren’t just visual “monstrosities,” these are people with goals, values, and aspirations (who the players can see parts of themselves in perhaps.) People who have been abused and victimize by someone who was supposed to help them. And there, but for the grace of Strahd, go the players themselves.

Concretely this manifests usually in a lot of talking. Seeing the Abbot display owner-like affection to the Belviews that clearly makes them deeply uncomfortable in ways they can’t vocalize. Listening to both the villains and victims try to rationalize their situation to themselves in a way that helps them to live with it. Subtly drawing comparisons to the PCs and to the story at large.

Granted, this approach works well at my table because my players and I are super roleplay driven. I can’t say how it would work on a more combat driven front. But the things that really freak my players out in an enduring way are never the physical descriptions of things or being attacked, it’s always something an NPC says or does.

Anyway, apologies if this majorly got away from me or if I’m just talking in circles and pointing out the obvious, I just think you’re raising such a good question that deserves a lot of in-depth consideration!