I am not a physician, and so encourage you to check out any of my comments either with your physician or perhaps to get a second doctor's opinion. I am a type 1 diabetic of 34 years, and so I will answer you from my own base of knowledge. People who are not diabetic or hypoglycemic DO normally have blood sugar variations throughout the day. How much the sugar varies from before meal to after meal seems to be the issue here. The glucose tolerance test is the correct test to see how your body reacts to high levels of carbohydrates. It does sound to me as if you perhaps vary more than is ideal, for a low of 57 or high of 256 is something that even diabetics try to avoid. From what I have read, the high-protein low-carbohydrate diet seems to be the best solution to people who have over-reactive pancreases, and this may to be what is happening to you. When your sugar rises above 200, your body is going into panic mode and is producing too much insulin, which then brings your glucose readings down too low. From what I have read on the subject, hypoglycemia is fairly common as a reaction while a person is adjusting to the low carb diet, but then things level off and the person's blod sugar stabilizes. I would recommend that you stick with the diet, but that you become very attuned to when you need a small snack to bring up glucose levels until your body stops over-producing insulin. A VERY SMALL amount of juice (absolutely no more than 1/2 cup) will bring your glucose levels back up within a few minutes, and then some protein such as nuts will keep you stable. From what I have read, the low carb diet is ideal for people like you, although if hypoglycemia persists, you may find that you need to increase the carb count in whichever meal precedes the usual glucose drop by a small amount until the drop ceases to happen. I encourage you to do some reading on how low carb diets work.
I think you can't judge from the results with finger picks. They are not reliable. The most reliable result should be a fasting glucose done in the lab, with blood drawn from the veins, but not from capillaries in the fingers. It's not useful to just have finger picks randomly. If you have finger pick within 2 hrs after meal the result will be high even for normal people. 4 hr OGTT is not very reliable
From what I've known, you can only say someone is diabetic if there is one of the following:
1. fasting plasma glucose >126 mg/dl
2. random plasma glucose (>2 gr after meal) >200 mg/dl
But as I've said, the most reliable one is the fasting plasma glucose
As a supplement, are you sure it's 4 hr OGTT or a 2-hr one? 2 hr OGTT is only done when the fasting glucose is normal but the clinical picture still points to diabetes. As i've said 2 hr OGTT has a low reproducibiliy, but that's the only test we can do if the fasting glucose is normal but diabetes is still suspected. I think it's likely that you're receiving proper care, though I don't really know about the details of your situation. You have to be very unfortunately to meet a physician who can't even manage simple things like diabetes.