sometime too they say that the dry kibble can go stale like crackers in a pantry, so maybe the hot water is helping the smell come out more to entice her to eat up better.
Brew was soooo picky too. i bought like 10 different types hoping to find the one he would love day to day. and i too was heating up the food to get him to eat. plus searching yorkie talk daily and posting all sorts of threads looking for help as well.
i have no idea what triggered the big change in his eating habits (neuter, 3 baby teeth out, turning 7 months old, fasting for neuter...) but he is now eating everything we put out for him.
he gets either FROMM: duck & sweet potato kibble or taste of the wild: bison & venison kibbble for breaky
and wellness canned foods in the evening: whitefish, venison or lamb with sweet potatoe...to answer your original question.
but all yorkies are different so what works for us may not for him.
and hopefully i didn't add to the confusion!!