What is your daughter doing for 2-3 hours between waking up and eating? I get that she's not hungry, but she's doing something. I'd say that eating is required before she engages in anything other than going to the bathroom and perhaps getting dressed. She doesn't need a full meal, but she must have protein and some sort of healthy, long-lasting fat. A piece of string cheese, a scrambled egg, 4-6 ounces of yogurt, etc. No plain bagel, no sugar cereal alone. A quick 4 ounce glass of OJ might work in part, but a few sections of clementine or orange would be better (fiber and real fruit), and then immediate protein on top of that juice. She probably doesn't want oatmeal in the Florida summer heat, but that's a good thought going forward, especially with fruit on it. She may need to be a "grazer" with smaller meals/snacks, but they have to have some fat and protein in them. Fruit and veggies are okay, but they don't last long, and the fruit gives a quick sugar boost followed by a crash. Better if they have something extra: veggies dipped in substantial amounts of hummus, apples or celery with a good slather of peanut butter on them, etc.
Some of this is behavioral management - while you don't want to punish her, you don't want to give her 2-3 hours of sitting around, watching TV, using the computer, and getting so distracted that she forgets to eat. She has to eat. Period. She has symptoms and she cannot be allowed to ignore them.
Just because an underlying cause isn't found, doesn't mean there isn't one, you know? There are epigenetic changes in everybody, and sometimes those affect the blood sugar levels while in another person they cause digestive issues or even behavioral issues. So don't focus as much on the causes right now as on the management of the symptoms. I work in food science and we just give our kids (and ourselves) a balanced and patented shake of essential nutrients. If a kid only has half of it to start, fine. It can be finished later. It balances out a lot of blood sugar issues, but is also helps kids without that issue just get a good start to their day. Building healthy habits is really important.