Well, I can't speak to the evidence to or against the protocol for massive insulin overdose. Family members of diabetics are drilled on how to deal with hypoglycemic events, certainly moderate insulin overdoses (from e.g. underestimating the bolus for a meal) just come with the territory and that's about all I'm equipped to deal with. I think the protocol is simply based on: low brain blood sugar = seizure/brain damage/coma/death, avoid at all cost. Just based on that, delivering sugar directly to the bloodstream makes sense.
I'll point one relevant complication: diabetics often have neuropathy, sometimes this neuropathy affects the digestive system. This is a doubly cruel issue: low blood sugar causes neuropathy in the long term, and if it affects the digestive system then trying to ward off low blood sugar simply by eating is hard. I doubt protocol assumes potential stomach nerve damage in all patients but it's something worth considering.
We do have some experience with hospital diabetic protocol: they err heavily on the side of keeping your blood sugar high, like over 150 mg/dL high. And yes, you can still run into nurses who are ill-equipped to deal with diabetic patients - we ran into one who refused to push dextrose even though my wife's blood sugar hit 15 (I have no idea how she was still able to argue with the nurse at that point).
Re: your question on glucagon - as I mentioned elsewhere in this thread, yes, the liver glucagon cache depends heavily on your last meal and exercise level. I believe it fluctuates from 50-200 g or so. If you've just run a marathon, a glucagon shot is not going to help you.