Canceling the surgery is definitely the best thing to do.
Given the latest update, though, I'm not convinced an Ophthalmologist is an essential next option. If her appetite slacked off a few days before the eye problem, and now she's having trouble chewing, I have to suspect a tooth issue. That is I believe the most common cause of Exophthalmos in Lionheads.
You needto cancel the surgery, but you may want to either consult with your current vet (especially if he'll give you a free consultto re-assess) and gofor aset ofx-rays, or find another very rabbit-savvy vet in your areaexperienced in rabbit dental issues.
Your current vet's reasoning may be that the cheapest option was to operate and just diagnose and deal with the cause during surgery seeing as x-rays may not be conclusive, either, but eye surgery on a rabbit should not be the first option.
I've sent out queries to a fewprofessionals to see if they agree, so hopefully I'll have more info shortly.
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