You're not in the mega dangerous territory with your HbA1c or your fasting blood glucose level. They will become lower with some work.
How are you managing your type 2 Diabetes?
I think your anxiety related to the potential to hypo has made you overreact to avoid one.
The banana would probably have been sufficient on it's own. I'm not surprised your blood glucose rose so swiftly in that 15 mins! :O
Are you able to describe how you felt both physically and mentally before you ate all those carbohydrates as a preventative measure?
I'm fairly confident, as you say you're on 1 Metformin a day, that your Liver will be capable of avoiding a full blown hypo for you. You may feel 'uncomfortable' when experiencing the low blood glucose levels but, if you test regularly at that time you'd probably see your body sort it all out for you on it's own.
I'd suggest you should only really start to feel concerned when you go under 4 mmol / L - "fours the flour" is a very misleading phrase that is used out of context a hell of a lot as how we all feel at various blood glucose levels is a personal experience.
I hope the web searching for normal blood glucose levels has allayed your fears somewhat.
Another factor to consider is that your blood glucose meter could be out by up to 20%.
So, if it reads 4.6 mmol / L you could actually be anywhere between 3.68 mmol / L and 5.52 mmol / L however, they do tend to be more accurate at the lower figures.