If casein cannot be tolerated at all, then the only dairy products I've seen that are okay are certified casein-free ghee (don't know where to buy it, I've just seen references that it exists), and butter oil (the high-price supplement for vitK2 X-factor).
We need to avoid all casein, and these are the only things I'd consider (but I haven't purchased, partially because I don't know where to get that kind of ghee, and partially due to price).
I think goat's milk has less casein than cow's, and I _think_ fermenting can change some of the casein (I think it breaks down some), but not all.
I have gotten the impression that unless you absolutely need to remove all casein, the only way to figure out what small amounts are okay is to experiment. Goat's milk in general, and fermented products in general, seem to be better tolerated, but it really seems individual as to what's okay and what's not. I don't know if there's a better way to go about it than to just try.