I'm guessing you've been round all the special diet options by now?

If the painkillers make Little feel better, then that is a good thing - Google suggests that in the USA they have a Fentanyl patch that dispenses a strong painkiller slowly via skin absorption. It might be worth asking the vet about that as easier and more effective than pills?

If she feels better, maybe she will drink and eat a little more - keeping her comfortable seems like the main goal IMHO

*hugs*