I can't quite understand what you are seeing in Bentley's eyes so a photo, as Karen suggested, would be helpful. Are the cone shapes of what appears to come from the pupil "invading" the iris or the cornea? Your vet might have seen changes at the back of the eye, in the retina. What did you find online that showed changes to the eye? I thought the changes a vet would see to indicate FIP can only be seen with an ophthalmoscope.

When did you notice the cone shapes? Did Bentley have a URI a few weeks before you notice the change? As you now, FIP comes from the coronavirus but if you had the other two cats at the same time as your cat who died from FIP, they could carry FIP and shed it.

I do empathise with you. I lost three foster kittens from a litter of nine to FIP right after they went out for adoption. They were bounding with health, got colds at the adoption center and then developed FIP. It's a truly horrible disease.