Lets be clear about a few things.
FIV & FIP are not the same things.
I've seen both used in this thread.
It's really not a question of testing positive for Corona.
ANY cat that has been exposed to it will show a titer on the test. This doesn't mean that it's suffering from it, only that it's been exposed. I would venture to say that most every cat will show a titer, no matter how carefull anyone is.
She is definately a breeder, and she shows in at least one association, if you really wanted to you could find out which one just by going to the association sites and checking points on cats.
On her website, she clearly states that all kittens will have a contract, if you didn't get one, i'd be asking why not.
A corona virus can be VERY devistating on a young kitten, doesn't mean that it has FIP, unless there are clinical signs that it either has the dry kind or the wet kind. It's inability to walk could have been because of dehydration.
Perhaps I missed it, but did you get another opinion? As i've said before, you can always rely on a vet, they don't know everything.
I have to agree with Maui, be carefull about what you write, it could come back to bite you by the breeder.