Is it possible for someone to have an abnormally high plasma glucose concentration but not have glycosuria?
Yes. The threshold for glycosuria is blood sugar of around 200 so the blood test is more sensitive for detecting diabetes.
Yes, Glucose only spills into the urine through the kidney filtering mechanism when the plasma glucose is persistently over 180 mg / dl. This is well n excess of the blood plasma levels necessary to diagnose diabetes mellitus.