See if you can get her to switch to smoothies and pediasure, or milkshakes with protein powder. Until your doctor gets a grip on what is causing it, smoothies and pediasure both give her calories and protein without having to 'eat'.
Miralax, if I remember correctly, just pulls water into the intestine.
Once it's out of the system, diarrhea shouldn't be an issue. There are several stool softeners that are much less 'effective' at pulling water, while still avoiding hardness issues... or give a modified diet (no rice, lots of fruits, vegetables, and water) a longer try - watermelon or prunes and warm liqid (soup, tea, coffee, warm water) is the most effective - gentle - predictable laxative I've ever seen.