Hi anniemack,
First, I must give the usual disclaimer: I am the mom of a diabetic teen, not a doctor. Your daughter's symptoms sound complicated and frustrating, and I understand your worries.
As for the question about "normal" people have blood-glucose readings of 200 or higher, no, those numbers are higher than normal. They're not as high as those of a typical newly diagnosed type-1 diabetic though. My daughter was at 450 when she was diagnosed at 14, and I know some who were 600, 800 or worse.
Something is going on, but it might not be diabetes. If it were diabetes, she would be very thirsty and urinating a lot, and losing weight. It doesn't sound like she has all the classic symptoms. As for what else her symptoms might mean, I couldn't guess, because diabetes is all I deal with. Hopefully the endocrinologist will get you some answers and some treatment. Hang in there... I'm sure she'll get better.
Has she seen a gastroenterologist? A number of the symptoms you mention can be associated with inflammatory bowel disease (ulcerative colitis or Crohn's disease), including cessation of growth, susceptibility to infection, stomach pains, discomfort in eating, and constipation or diarrhea (or both). Just a thought. I hope you find the answer soon.
I forgot to say, GERD is also very common with inflammatory bowel disease.