The weight loss forum may be better suited for your post.
The only way to know if you can manage your diabetes with diet is to monitor your blood sugars fasting and 2 hours after eating. YOur doctor can also test your HA1c to give you an idea of how your blood sugars have been over the past few months.
Some people do find that metformin makes it easier to lose weight. SO, yes, there can be a connection.
Also, the last 20 lbs is often very hard to lose. The most effective way to lose it is to continue with your diet, but add in some extra exercise.