In my opinion, good control of CAH would be all that is needed to avoid diabetic symptoms such as insulin resistance.  Due to the way the cortisol is metabolised (quickly for some) this in itself affects the blood sugar and insulin sensitivity etc.  I know that when we switched Ashley to Dexamethasone this problem of high and low blood sugar and mood swings resolved itself magically.  It is the bioavailability of hydrocortisone I feel that predisposes a child to diabetic symptoms.   The drug is too short lasting for some kids and this is what causes the blood sugar swings etc. 

The child is NOT diabetic in other words - it is side effects of the drug or should I say side effects of the drug and the side effects of the lack of the drug in other periods.  Not to mention that you can help yourself with the right diet and understanding what foods are best not being eaten.  Once those two problems are resolved to my mind you would not need metformin which is just another drug with possible side effects.  I’m not suggesting a switch to dex for your child or self.  There are other long lasting steroids such as Pred that can be utilised at certain times of day that would work just as effectively and can be out of the system before the child sleeps.