You cannot make anyone do anything. I know that is hard to hear, but nobody can make your grandfather eat right, test his blood sugar and monitor his insulin but himself. He is an adult who maybe needs to hear how much you care and how scared it made you when he gets like that. You can help and be supportive of course which you seem to be doing.
The high of 399 could have been some juice on his fingers. If the hands are not clean, and have a sugary residue, then it will not be a correct reading.
He seems like he was hypoglycemic, and that was just a false reading. I am sorry I do not have any other advice but I guess you will have to be patient and persistent with your grandfather. It is very hard to be diabetic and if he got diabetes later on in life it is even harder to change your ways.
It's great that you are making the effort to find out more information about your grandfathers disease. My husband of 12 yrs has been Type 1 for most of his life. The episode you witnessed with your grandmother is scary, it's scary to try and help someone in that situation, as well. Many times my husband does not remember what happened, what he said or what he did. Not fully, anyway.
You don't say how long your grandfather has had his diabetes. It sounds like your grandmother did a good job of keeping the situation under control.
My husband has had these low blood sugars, and he's reacted in similar fashion, sometimes worse, sometimes it's not too bad. We usually discuss what happened and what we can do to correct it. Eating on time, eating more/giving less insulin when he's done a lot of physical activity. If your grandfather is stubborn and doesn't want to make positive changes, that could be a problem. Everyone deals with their disease differently. Maybe he would listen to your concerns about his low blood sugars? Maybe a chat with his Endo would find that he could make an adjustment to correct a dosage error?
You're not alone with the challenges of caring for someone with diabetes. I believe the diabetic has to want to be safe and healthy. If they don't want to take the time to test, make the changes to their diet or think before taking insulin, you can't make them. They have to care about themselves before you can help them care for themselves. I know that doesn't solve your problem, and it may take time. But keep learning about the disease and be there for your grandmother. She's in a tough position, I'm sure she'll appreciate your support.
This site is a great for seeing how others deal with their disease -- the good and the bad.
Good luck.