The crux of this quesion appears to be "when the engine is running at low power in a descent how can it produce enough heat to prevent carb icing?"
The short answer is that it cannot.
The thing to rememeber is that most carb heat systems draw their heat from the exhaust manifold. In a long descent the exhaust manifold will cool down, eventually making carb heat ineffective. This can be prevented by increasing power periodically to warm up the exhaust manifold.