Oh, Robin, I'm so sorry that you sound just like me!
I was diagnosed with reactive hypoglycemia at 16 because my father has it, too. Simple diagnosis, simple treatment (low carb), right? Wrong. Though I had a "normal" OGTT, within 6 months I started to see highs. They've gotten worse over the years and when I started to spike to near 200 on a regular basis (almost always falling below 140 by the 2 hour mark, but this happened several times a day), my endo decided it was an enzyme deficiency and pulled me off of grains, which helped some. (I think that's because I'm not eating many carbs at this point.)
Have you had a c-peptide done? What about an insulin level? Those are important to see whether you're insulin resistant or low on insulin. My c-peptide is 9.7, where the normal range is between 2 and 4. I'm young and thin and active and eat well--no way I'm THAT insulin resistant. I'm going to bring that up with my endo when I see him at the end of February.
I've heard of reactive hypoglycemia being the extremely early stages of LADA (and yes, sometimes if it's very early those tests may not come back positive for LADA), but I've also heard of it being the early stages of T2. I personally have had 2 reactive hypos in the last few weeks but otherwise haven't had them for months. What everyone has said about over-compensation by the pancreas for high blood sugar is absolutely true, and I have the stories to prove it.
My family also has some autoimmune floating around in it, but I have more immune issues than everyone in my family put together. Yay for me! My antibodies (as of the summer of 2011) are also normal. Go figure.
As far as being tired is concerned, there could be a few things at play. One could easily be Hashimoto's disease (autoimmune low thyroid), which I happen to have and would fit in with your family's history of autoimmune. Another could be that interrupted sleep you were talking about--interrupted sleep is pretty exhausting. A third could be the blood sugar swings. On any given day, if I've been low or high, I'm usually left with a pretty bad migraine and exhaustion that doesn't go away even after I treat the migraine. The worst are days when the differences in BG could stand as blood sugars on their own (like a day I had last week where I was 174 and then 63 an hour later--that's a difference of 111 mg/dl, which could stand by itself as a rather nice blood sugar).
Finally, an A1c will only show highs or lows if you spend a significant amount of time there. Being 275 2 hours after a meal sounds pretty high, but if it happens once a day and let's say you drop within the hour, that's 3 hours of being high and 21 hours of being normal--so that's only about 13% of your day being high. Though that can certainly make you miserable, it won't have a large impact on your A1c, especially if you hang out under 120 for the rest of the day. My A1c is also normal--I think my latest was 5.1, despite the fact that I was seeing multiple highs above 160 every day for months before it was drawn.
I wish you the best of luck, and may you search for answers for less time than I have!