Ah, darn it! Would have been the easiest answer I guess!
Instead of cutting back her food, have you tried her on a lower protein diet? I have one girl who has issues with high protein diets (she can't tolerate the grainless kibbles or starts having accidents as well). Just another thought - sorry this is so frustrating for you, but it seems like you guys have made strides with her - I'm sure that this was just a back slide and that it will only get better.