If they are treated properly, do people with mild CHF die early?

No. Life expectancy with CHF has made huge strides over the last 2 decades. Prognosis is better than it's ever been. If there is a reversible cause, it should be pursued, of course. Even with advanced chf, cardiac resynchronization, intracardiac defibrillators, and lv assist devices have made a huge difference. Current drugs often can improve heart strength also.
CHF. Patients with any form of congestive heart failure are at an increased risk of dying sooner than if they never had heart disease. Of course, there are many factors involved with these projections including the cause of the heart failure, how well it is controlled, the age of the patient, other medical illnesses that you may have -- just to name a few.