Hey!
When the car reach a certain temp, the coolant flows into the coolant reservoir overflow tank.
When the engine has been turned off, and cool down, the coolant will flow back into the radiator.
My question, if I keep changing the Coolant in the Overflow Tank frequently, does that keep the coolant system clean? instead of keep flushing the entire coolant system!
No.
No. The cooling system should be flushed properly.
No, most assuredly not. Check how much your cooling systems holds versus what the expansion tank holds.
No. That tank is one of the highest points in the system.
Debris and crud that forms in the engine over time is heavier than the coolant, and settles toward the lowest levels of the system (bottom of coolant journals, radiator bottom, etc). If it's relatively fine, it may move around when the water pump is going, but as soon as you turn it off, it settles where it is. That overflow only has a small fraction of the total coolant, and generally doesn't have much flow in and out. Stuff will settle in there, but only if it's really bad everywhere else.
