I had a diabetic dog for five years, I would not hesitate to ever get a second opinion.
There is always very good advice on this site but much of it may not be helpful for your pup.
I had on occasion done the chicken/rice or burger/rice boiled but I never gave my diabetic dog white rice, way to high in carbs for him, I always used brown rice as it's a complex carb. He tolerated it very well and it did help with his diarrhea but that does not mean it will help you dog.
Twelve is not necessarily an end of life time either but how long has your pup been diabetic?
I wish I could offer more but you need to work more with a vet than with online sources no matter how much we all would love to be able to help you.
Good luck and please keep us updated.
Sher